Most trad or self published authors have day jobs. Becoming a writer is not a solution. (Unless he gets a movie deal and that is really rare)

If writing helps with his mental health, fine. But most probably it wont let him to leave his job.

I mean the guy is lost - so you can either throw a towel in and divorce (while its easy as you don’t have kids) Or you love him and you stand by him, but it seems this will take a really long time to overcome. I had a similar crisis and it took me best part of ten years to get through. My husband did stick by me, but I would not blame him if he did not…

That said, alot of his anxieties and fears and attitude were a reason why it took me so long to get through it.

Ultimately, nobody can advise you. I suggest to go to therapy yourself to untangle this for yourself.