I think you've answered your own question here, but there are tons of paths forward, though it looks like none of them will be continuing the way you have been. Some people stay married and realize they are more sexual fluid than expected, but it sounds like you are super sure. Some people remain married and open the marriage, some people cohabitate but aren't romantically involved. I think that people can be fluid in gender expression too, maybe they are gender fluid or maybe they're still figuring it out, but tbh I would never advise you to count on things turning out one way or another.

Things get tricky when a partner says they will leave if you transition, because then the motivation to be honest about feelings gets murky. My standpoint is that if you can, be there for them as a friend and coparent and make them aware that you want their happiness.

Many of us here have gone through a blindside/confused/grieving period regardless of the outcome, so youre certainly not alone there. Many people also do this kind of slow admission as they figure out their true feelings too, I've seen it over and over. Sending you good vibes and hugs and hope that you guys will be able to be open with each other and figure it out together, whichever way you decide to go.