My DH is a good Dad. He spent years fighting for SS11, paid CS on time and in full every single month, and has tried to stay as involved as he can. Our BM has spreading the narrative of "Deadbeat Dad" since SS was born, despite her being the only barrier to DH's full involvement. 

My DH was (and is still to a point) abused by his ex/HCBM. They were only together as teenagers and har SS when they were 19. BM has obvious mental health conditions along with noticable narcissism. DH tried to "coparent" with her for almost 5 years. It was constant - BM not sharing information, BM informing DH of important events the day of and then bashing him for being unable to attend, several lengthy and abusive emails daily belittling everything from his work to his family to his housing to his finances to his parenting. When DH would try to put boundaries in place, telling BM that certain topics were inappropriate and off limits, BM would argue that they weren't because it all "affects" SS and therefore she has a right to be involved in it. She felt that she had to right to meddle in every aspect of DH's life. When DH put boundaries in place, suddenly SS was "no longer comfortable" visiting and BM would withhold visitation. BM has alienated SS to a point and made SS feel like BM and her family are SS' only family and DH is nobody. SS still loves DH and knows he is his Dad, but BM's behavior has made an impact to where SS does not see DH as a parent or a significant part of his life. It became harder when BM moved SS 4 hours away, limiting our visitation to 6 weeks per year. It felt impossible to deal with BM due to her constant behaviors. She was never wrong, was a pro at gaslighting and making us feel like we were at fault or terrible, always had to have her way and being full control, etc. Oh, and don't even get me started about the DOUBLE STANDARDS. It was like talking to a brick wall until we realized that BM was never going to change, so we had to change how we dealt with and reacted to her. 

My DH took BM to court in 2019 to try to stop the abuse. While a judge did tell BM she was wrong for her meddling and over involvement in DH's business, the judge did not grant DH more time or access to SS, therefore giving BM what she wanted. The court is allowing BM to continue her abuse through limiting visitation. Once court concluded, DH made the conscious decision to essentially stop fighting. We have visitation and call SS on the phone as the CO orders, we respond using BIFF method to BM, and we pay CS (not a penny more). I stopped all communication with BM - text, email, seeing her at pick ups, etc. DH doesn't try to get overly involved with SS' stuff unless he can access it himself. He reaches out to school, sports, and other entities directly to obtain information and if he can't do that, due to lack of information from BM, we just leave it alone. When BM emails DH, he responds "thank you for the update" or keeps it as short and businesslike as possible. BM HATED this in the beginning and tried everything she could to get DH to engage with her or react, but it didn't work. DH stood strong and now, BM rarely engages with him. BM still does petty and stupid things out of spite, but it isn't nearly as bad as it used to be. 

I recommend DH seek counseling. It was the only thing that gave DH the power and understanding that he wasn't wrong and that he was entitled to emotional and psychological safety from BM. This empowered him to create boundaries that he felt he couldn't have due to losing SS. It also gave DH "permission" to give up fighting for SS, since it was a fight he would never win. For you, seek therapy, vent on here, block BM on all medias including text, email, social media, etc. Do not give BM the headspace she doesn't deserve - it is hard, but must be done to save your sanity. 

an extent that removes any benefits they get from bullying.

A bully X, or SO's X, is no different than dealing with a bully in childhood.  Being bullied hurts. It is going to hurt whether we confront the bully or not, so send the bully to the hospital bleeding and in need of reconstructive surgery. If they do it again, send them back for more surgery.  Figuratively of course. Unless they make the mistake of attempting to be a physical bully on top of being a philisophical bully. In which case, they choose to need the reconstructive surgery.  If they limit their bullying to the philosophial, destroy them legally, financially, socially and with their children.  

Kids need the facts associate with a toxic parent's toxicity. Those facts provide a kid with the tools needed to protect themselves from a toxic parent.  Not only as they are growing up, but also so they can protect themselves in adulthood.

A toxic parent or toxic half of a blended family equation is not going to just stop being toxic because a kid reaches adulthood.  The odds of that are slim and none.

IMHO of course.

If your DH will actually do anything for his daughter, it is time for him to find some testicular fortitude and destroy the X breeding partner BM.  Protect his daughter. By ending the X legally, financially, socially and using any effective and not illegal means necessary. 

Time for him to stop talking about doing anything to benefit his daughter and actually man up.

I had my DW's back for our 16+ year battle with the SpermClan.  But she was the one that destroyed them one idiot SpermClan move at a time.  We met when SS-30 was 15mos old and married the week before he turned 2yo.  Looooong story shorter, he is a man of honor, character, and quality who is a successful adult and of standing in his community and profession.  His SpermClan? Not one of them is even close.  His 3 younger also out of wedlock Spermidiot spawned half sibs include one on the dole, one in prison, and the third not far behind the inmate.  

My DW, truly did deliver on her promise to do anything for her son, to raise him to viable adulthood, and to protect him from the shallow and polluted end of his gene pool.

Just my thoughts of course.

There is a playbook that HCBMs all seem to follow. 1) They are only ever looking out for the "best interest of the child(ren)". 2) They are always the victim and dad is just bitter and out to get them. 3) They can lie with a straight face and often do to anyone who will listen to feed into the narrative in #2. 4) No matter what dad does (give BM everything she wants, ignores BM, follows the parenting plan exactly, etc) he will always be "wrong" and "difficult", according to BM because he "doesn't care enough about his kids". 

My DH was also a complete pushover when it came to BM when I first met him. I think he had successfully convinced himself that she was not abusive, despite the fact that multiple counselors had told him that she was...including the marriage counselor they say before they finalized their divorce. In my case, this was because DH has a domineering mother who also has abusive tendencies, so to him that behavior was normal. He also didn't believe that men could be abused. That could be behind some of your DH's behavior. It could also be embarrassment that he's found himself in this situation or guilt that leads him to feel like he deserves it. That doesn't excuse his behavior and I think your reaction is normal. In fact, a counselor DH and I saw before we were married (because BM really kicked up a bunch of drama after we got engaged), asked me if I had a difficult time respecting DH when I watched him fold to BM's demands. 

As far as how I/we handle it, I have gone full no contact with BM. I am not in the same room with her if I can help it and if I am, I completely ignore her. I don't have time for that BS and I don't see it as my responsiblity to validate another woman. DH basically ignores and avoids her, too. Skids are now 17 and we rarely hear from her, unless she wants to cause drama, but that happens less frequently now that Skids come around less. The next drama fest will be demanding that DH pay for college...even though neither kid has any business going to college. 

To a certain extent.. you have to be able to disengage from his EX.  While you can be supportive of him.. he also may feel that the juice is not worth the squeeze by taking too hard of a line with his EX.  Because, barring horrific action on her part.. the likelihood of her having her parenting cut back is probably slim.  I'm not sure what you mean by "more parenting".. whether he is looking for 5050.. or whether they did not have an official order and he is attempting to get that in place.  The unfortunate reality is that men still are often handed a shorter stick with the courts.. even when the mother is a less than stellar human being.

So, you can set boundaries for your home... for your personal financial involvement.  But, your DH is a big boy.. he had a relationship with her and now he has to deal with the fallout that resulted from them having a child together.. you can't protect him.. and you can't prevent her from being high conflict.. in most cases.. there is little bite to anything that could be done.  And.. as you know.. if he pushes too hard back at her.. she cuts off access to his child.. so.. he is likely walking that tight line of letting her "run her mouth".. but not taking it personally and doing it so he can see his child.

Now.. if her treatment of him makes him act or behave differently towards you?  in your home? if his acquiescence to his EX causes issues in your home? those are lines you draw with him. And.. maybe part of what needs to happen is that he just not share every aggravating thing she says.. because it isn't productive.

Get ready for a long hard and expensive road.

Good luck!

Id say disengage. You are not alone, we have all gone down the resentment rabbit hole. 8 plus years and Im still going down it. Ill have to post!

Don't "handle" the ex, let the spouse / partner do it because it's too much wasted energy on your part  

Do hold FIRM boundaries!

Like mentioned above, disengagement helps too.