Please don't marry this man.  You need to protect your finances and not let what you own become "marital property."  

(I edited this remark after I wrote my initial post.)

I hope you will game out what your response will be if he tries to breach the boundary you set, as he is likely not to respect it, or to manipulate you in some way to set it aside.  My ex used the old "poor me, this is so hard, won't you help/comfort me?" move, and because I am vulnerable to that sort of appeal, I caved.  So think about your own areas of vulnerability, and be on guard.  

I'd say AGP is the driving force behind his move to "gender fluidity"; I'd also say that many many of us here have experienced a cascading sequence of "what our spouses say they are" that changes over time.  A gay man in denial might say he's bi, or that he's emotionally attracted to his wife but physically attracted to men.  A man who will end up calling themselves trans might begin by calling himself "non-binary" or "gender fluid." I have said here before that the old joke of trans identified males themselves is "six months from cross-dresser to trans."  

I personally don't buy the defense of "I cross dress to relax" or "I cross dress for comfort" or "I cross dress because I like the feel of women's clothes."  If a man just likes the "smooth" feeling of women's lingerie, he could buy a silk robe or boxers and get the same "feeling."   If he does it "to relax," what about "wearing women's clothes" is relaxing?  As for comfort?  Very few women's clothes of the type cross dressing men usually choose are designed for comfort.   

Although they may try to normalize it by claiming those things (that it is "relaxing" or "comfortable") choosing to cross dress is not a neutral act.  It is not a choice made in a vacuum.  It is not just one of the available paths one might take to relax or experience comfort.  Choosing to experience what they take to be woman's existence or experience is likewise not a neutral act.  It is profoundly interested.  What do you suppose he would say, should you ask him, which I don't su

Please don't beat yourself up.  You were not "stupid & blind."  You were blindsided by something that was so out of the blue and so out of character that you were unable to take it in, and then you were lied to by your spouse, who you had every reason--and need- to trust and believe.  You had children to raise and a home to keep together. And as time went by, you had more and more of an investment in your life.  

The closeted man is an expert at blame-shifting, minimizing, false equivalencies, manipulating, and gaslighting.  They make us doubt our reality, our sanity, our judgement, our attractiveness, our ability to discern the truth, all while not so subtly projecting their shortcomings and shifting the blame for them onto us.  We end up internalizing their criticism, and start doing their job for them. 

Instead of now blaming yourself, engaging in negative self-talk, I hope you can learn to talk back and reframe your situation.  That even after twenty-five years of his abuse of you--that secret sexual basement and his accompanying running down of you--you weren't crushed, and have been able to see through the curtain  of lies is a testament to your strength.  You were not crushed, but are now seeing clearly.  

I see others have said they would avoid mediation.  I agree.  

Also: Your attorney is correct that striking while your stbx is amenable is a good idea.  My ex at first was "I just want to do the right thing," but by the time we actually divorced, three years later, he had completely changed his tune.  

Yes, it is a shock when you see them all tarted up.  The good thing is that seeing him brought you to the point that you know you're doing the right thing, and that there really isn't any other reasonable course of action.  Based on my own experience of being confronted with my ex in full get-up, I wanted to say that you may have a delayed reaction to having seen the full crazy.  Which is another reason why you need your attorney, as s/he will remain clear headed, and is yet another reason mediation could be risky.  

Jupiter:

I had a late life divorce (I was 64) from my autogynephilic cross dressing trans identified now ex.  He, like your stbx, thought he should control the process and dictate what the settlement should be.  He also insisted on mediation, although in our state mediation was not applicable in our situation.  I went to the mediation just because he was so dead set on it, and while there the mediator told him exactly what I had.  

I was used to deferring to my ex, and standing up for myself was very difficult.  But I did it, although I did make a couple of decisions that gave him a better deal just so I could get the divorce over quickly. I also insisted he make a concession, too, and although he kicked, I knew the courts would back me up.

My advice is to have a consultation with a lawyer--get a good family law specialist--and find out what you are entitled to.  My ex had very clear ideas of what was "fair," but his ideas were not in fact what the law said was fair.  Once you realize that you have the legal establishment behind you, you are more confident.  Blue Bear is correct: you need your own lawyer.  

My other piece of advice is to jettison all these alphabet (LGBT+) therapists.  You should not be spending ONE second on trying to figure out where you stbx is in his "gender journey."  That's his job, not yours.  You are divorcing him, and I cannot stress enough that you need to stop "wife-ing" for him.  Nor do you need anyone who isn't solidly behind you.