You have a lot more details going on there then -- between teen son with challenges, and then DH's mental health challenges, and your own gender transition challenges.
Any one of those is a biggie, but so many at the same time. How is your stress? His? Do you do anything for that?
For me thinking about a third person in future has to start with YOU, and making yourself the best and healthiest you can be. Then working on making your rship with him the best and healthiest it can be.
Health people attract healthy people -- unhealthy people attract unhealthy people.
Maybe talk over the worksheets at opening up? http://openingup.net/resources/free-...om-opening-up/
Just as a touchpoint to start on?
See where you are all at and where you might need to go to strengthen skills?