I have friends who are in a similar situation, though younger. Together over a decade, two beautiful kids. Husband always identified as genderqueer but had a mental breakdown after first child was born because they couldn't breastfeed. Finally decided to pursue nonbinary transition. As soon as the transition started it has seemed to put the wife in an emotional tailspin.
Transition can be kind of selfish and self directed. They have long been the emotional support for her, and now she is watching them be so much happier and it draws attention to her own deep unhappiness. (She is very unhappy due to her abusive upbringing but despite cutting off contact with her parents due to a recent blowup she can't bring herself to accept they were abusive and keeps looking for another explanation to throw blame on. She decided to first self diagnose her son as ASD (he's not) and then herself (she's not).)
My wife has noted on her behavior in person and on facebook, and we discuss it. Our friend is bi and in no way no longer in love with her hus-wife or less physically attracted. But she is feeling very stressed emotionally by the transition and the shift in focus aware from herself and her needs and, I truly believe, the impressive calmness that overcame them after starting E that she can't seem to find for herself.
The worst thing I find is that she puts her own struggling to correct pronouns (for them and for me, who started hormones at the same exact time) on her older child. Guess what, I saw older child post op and his "rude questions" that she thought he would ask was all about my big owie and could he see and touch the bandages. (See, yes. Touch, h*** no.) Look, don't put it on your kid. They absorb this stuff better than adults. And there's no shame in flubbing pronouns sometimes if you've been in a years-long habit of using different ones.
Another example, unfortunately they have been getting harassed in public because Florida. So now she is claiming, oh this woman confronted me (about her spouse) because she thought I was mtf too. Ya right. Just emotionally flailing around she is. I think she truly feels bad and feels scared that her spouse is getting this scary, creepy crap. She is spunky and wants to be the defender, the warrior. But instead she is "normal", they are "weird". So she cannot take the role she desires. I feel bad for her but I worry about the path she is taking as well.
So it's totally possible your spouse is scared about: harassment for being a same sex couple, the fact that you are so focused on you maybe he is scared you will 'grow past him' (at one point a few years back I made some changes in my wife and my wife got scared of exactly this), maybe a little selfish pity party because you are focusing on you not him, or he can't figure out why he is sad, and jealous you have an 'answer' as if transitioning solves all personal problems (hint: it does not). I can't say you'll get through this intact but you do have hope, you CAN get through this as a couple if you're both willing and communicate.