CoffeeSonata I have not got any advice as I am not a trans widow but I wanted to mention one thing about fertility.....
"Our sex life is non-existent. It's never been 'normal.' He says it would be easier for him if he was dressed as a woman and was submissive." This strikes we as very selfish. I wonder if it would help you? I very much doubt it. Plus if you do want to have a family then (short of adoption) sex will be needed. I think it is very telling if you now do not want to have children with him. Please do listen to that inner voice.
"The support groups I found seem much more about working out how to stay and bend yourself to someone else's sexual identity and desires, without much consideration of your own. To say otherwise, or express this might be psychological, or a paraphilia is seen as transphobic." Stay well clear of any support group which doesn't actually support you! You can encourage him to go to the things that will support him.
"We don't have kids, but I was about to TTC (is complicated). Now I think that's a terrible idea, but time is not on my side. He is depressed and it's getting worse. I'm also now on anti-depressants. I just don't see an end to this... I love him, and feel guilty about even thinking of an escape plan..."
I think you really really need to think about yourself and what you want here. You want to have children, time is not on your side. My experience is of fertility issues, we had assisted conception, had a birth daughter and then had a lot more assisted conception, with donor eggs, all failed and we adopted. It took me a long time to get my longed for family.
Being a parent is fucking hard, my own kids have a few issues and I won't lie there are times when I wonder if I did the right thing even having kids! But I would not be without them.
If you want to have children and you know now that you do not want to have them with your husband then you need to think about yourself and where your future is heading.
Whatever happens you have not chosen this for your marriage. Your husband may also feel he has not chosen this, and he may pursue whatever help or support he wants or needs. But you do not need to stay together if this is not what you want. 