Hi! I'm also a full timer with a 16 ss and no bios. 
What has helped me personally was learning about teenage behavioral development and patterns. I spent a lot of time researching their brain development, reasoning milestones and all that stuff. For me, knowing what was normal behavior and what was not helped me have more patience and determine if something was personal or not.

I am also a full timer. My partner has ss13 and ss9. They sometimes go by the BF more the ss9 than ss13 but when they go for a few days it's like heaven. It's very difficult being with a woman who has kids. Honestly I'm almost 3 years in and knowing what I do now, I wouldn't get with a single mother. Now I am still working on disengaging from them but it's difficult especially with COVID and online schooling. They are always there...it's so annoying. Try focusing on yourself, your hobbies, what you like to do, what gives you peace of mind. I know we tend to get engrossed in our partner and try to help them  out but when it comes to her kids..they are just annoying brats who are lazy and halfway..I can't wait till they are older and out.

I am a full timer and I will be the first to admit I don't always cope well. My step child is 9 and she pretty badly behaved and I so understand the struggle of holding your tongue. I especially struggle when she is treating other people badly. Stuff like watching her tell her grandmother who has rearranged her entire christmas plans so she could see her that she doesn't want to come because the "presents are better at mums" and constantly telling her Dad he "looks like a girl" and ordering him around. I used to take on the burden of teaching her how to behave. But now I don't. I leave it to Dad and discuss it with him if I feel I need to do so. If I can't bare not to say anything I'll mention something to her afterwards like "How do you think that made him feel when you said that?" Or "If you are worried about Christmas, you can always talk to us." And then I leave it. A gentle prod. Nothing more. I've given her something to think about. It's not my job to reinforce discipline. As for family drama I've picked up new sayings. I used to help my husband a lot with what he should do when there was some sort of drama with the ex or his child, now when he comes to me, I say what I think but I always end it with something like - this is really your thing or this is really up to you or even hey this is your show. Making it very clear to him and reminding myself that the mess is his not mine.

I understand how awful it is being treated badly and the overwhelming feeling of needing to correct behaviour. I know I always worry if I don't correct behaviour I'll be dealing with it forever and my life will be ruined. But I think you are on to something with engaging depending on her attitude. Reward good behaviour.

I was a full timer for the 16+ years my SS-29 was subjected to the Custody/Visitation/Suport order.

It was fairly seamless for us.  He had limited long distance visitation with the toxic blended family opposition they they took intermittently. (7wks/yr - 5 summer, 1 winter, 1 spring)

DW and I were pretty much on the same page through it all and raised SS with standards of behavior and performance.  He is thriving in his adult life.

Partnership, standards of behavior and performance, and total confrontation of toxic bullshit from wherever and whoever it comes.

He knew who his dad is.  He asked me to adopt him when he was 22.  We made that happen.  He has put the shallow and toxic end of his gene pool far behind him and is living a good life.

I'm proud to be his dad and my bride's husband.  No blended family regrets.