I think that Acceptence and Commitment therapy would be the perfect therapy for both gender identity and mental health issues. It's commonly used to help those exact challenges, this is a book meant for professionals but if you look inside you can see the kinds of things it might help with
https://www.amazon.com/ACT-Gender-Identity-Comprehensive-Guide-ebook/dp/B07SX58VYL
https://en.wikipedia.org/wiki/Acceptance_and_commitment_therapy
It helps with your identity by helping you focus on your values, and committing to acting on them. And helps you develop flexibility so you can turn towards things that are bothering you, and commit to your values, and not have to turn away from them, which only increases your anxiety and gives power to them. I'm no expert I don't know what I'm saying really lol. But I strongly feel that working with a therapist trained in ACT (they all know it to some extent) with experience working with these issues will be really powerful for them.
As far as just helping them as a partner the book loving someone with bipolar disorder is really good, in the section for loved ones here
And the book A Liberated Mind is really good on ACT.
There has to be lots of people out there with the same challenges just not posting today, having some support and reaching out to some bipolar foundation or nami and asking around those kinds of organizations for ideas for supported or connecting you to others would probably bear fruit for some ideas as well.
Wish both of you health and happiness :)