I think it's good of your DH to offer to let you read the texts, but personally, I wouldn't, especially as there was some insecurity for you at the start of the relationship.   The texts go back to 3 yrs before she left him, and it would only upset you, probably, to read these.  I suspect if you did there would be one or two that would stick in your mind and cause you distress.  In your place I would thank DH for his offer, but decline. 

As someone who has done the "deep dive" into DH and BM's communications (only "co-parenting" communications, not relationship communications), do NOT do it to yourself. 

During our court battle in 2019, I read through hundreds of pages of emails and texts to help DH and his attorney prove a pattern of abuse and PAS. Reading through all of that caused almost a new level of PTSD in this steplife hell. I had to relive every horrible thing she did to us. Hell, I still think about some of the wording she used in her arguments to this day, and it has been three years. 

You guys are so close to being done with this horrible woman. I wouldn't subject yourself to any more pain or torture. Reading those texts won't prove anything you don't already know because you know that BM is terrible. If she wasn't, she wouldn't have alienated SS from you and gone to such extreme lengths to do it. I would let the past go and look to the future, working to find peace and heal from the trauma you have endured at the hands of BM. 

While I didn't have access to DHs texts, I did have access to his old emails looking for "court" stuff. Nothing from the "romance" error but right after the breakup. It was horrific to read. The things she said and did to my DH, a man who was still clearly in love with her and desperate to save the marriage at any costs. Heartbreaking. Crushing. And it explained a lot of his behaviors past divorce but damn.....I sure wish I hadn't been privy to that stuff. 

We are way past CS crap, BM has sinced died and all kids are into their 30's and 40's with minimal troubles these days but I'll never loose those memories of how abused my husband was within that first marriage and the vile woman BM really was, even before I came on the scene.

Do yourself a favor and don't read that crap. 

Put it this way, what could you possible gain from reading them that would enhance your life? Nothing, right? Don't do it, it isn't worth the potential pain. It's all in his past and is over and done with. 

Look to the future, not to the past ... always.

Ask yourself what good will come of you reading them. my guess is none so why put yourself into it. 

I can understand where you're coming from. Early in our relationship, I felt similarly about DH. ET was such a sh*tshow that it would have made sense for DH to go back to her, even if he didn't love her the same anymore. He would have had his kids back in his life, she would be crazy but at least a familiar crazy, he could have some control over his finances, and he'd still get laid (the thing I think ET regrets giving up the most).

It serves no one to go back and read through those texts. The only way you can prove to yourself that you're better is to believe it in yourself (as sappy and cliche as that may sound). A broken clock is right twice a day, so there will obviously be times when BM wasn't a total witch (she was a baby at one point after all).

And is it that you want to prove yourself better or want some sort of clarification as to why your DH fell for her in the first place? That has been a real struggle point for me. Like, how could DH go from her to me? Did his tastes change and he grew up, or am I actually closer in personality to her than I'd like to admit?

Hate to be the bearer of bad news, but it's often both. Likely you share some personality traits with BM (take a deep breath right here). Now, those traits may be her best and your worst, or they may be in the same universe but vastly different planets (e.g. you both like a good cocktail, but you know to drink ONE good cocktail while she chases her 7th one with a line of coke). And you'll probably find that your DH and XH share similar traits, too. Doesn't mean they are the same person or you made the same mistake a second time. Just means you know what you want but have refined it over time - same with your DH.

Don't look at those texts. Youll only set yourself up to feel worse because there WILL be good ones and you'll only fixate on those. You expect BM to be a witch, so you won't notice the hundreds of her calling him names, but the "I love you" will stick out like it was said a thousand times. It's similar to getting compliments and criticisms: every criticism takes 10 compliments to sting less. You'll just put yourself in the same mental space as that. You already know the outcome: she's awful, ruined your DH, ruined SS, and will ruin the next man who wanders into her life. The same bait-and-switch she used with DH and her XH#1 will be the same she uses for the next guy. None of those messages will be genuine, and even if they are, they're from a person who doesn't exist anymore to another person who doesn't exist anymore. 

It would hurt my heart to read old texts from my SO to his ex when they were together, so personally I wouldn't. 

Although I would be tempted to do it also, and may actually do it if it were me....I know I would wish I hadn't. No good can come from comparing. The most secure and happy you will feel is knowing how much DH loves you, and knowing it's over with BM. 

We always have that nagging feeling of did they love first SO more then you.  You are the second choice, ( fact)  You want to be love more then the ex.  They had years of time before the kids to be by themselves doing what ever (fact) 

I would not want a deep dive into crazy. 

Yep don't go there! It's horrible. I accidentally stumbled on old pics out of DH phone and there was one of Toxic BM sitting on his bed and he took a selfie with his face looking at her and it was how he looks at me. Oh man it took me years to get over that. No good delving into the past. 

Psychologically, when you read something like that, our minds and bodies feel as though whatever you're reading, even though it happened years ago, it feels as though it's happening now. Your body will go through a response similar to being cheated on. Even though we know logically that isn't the case.

You two are happy so there's no information you need. He's open about it so he isn't hiding anything. You need to think of yourself with this one. Reading it would be upsetting no matter what. Choosing to not read it would be choosing to protect yourself. Don't put yourself through the increased heart rate, flooded negative feelings, pain, comparing yourself.... There is a chance something could stand out to you and bother you, and it could act as a poison to your relationship. 

Your DH isn't doing anything so you should protect yourself.