My language has formal and informal ways of addressing somebody as well. Formal is the go to when you don't know a person yet, and in certain settings (doctors, teachers, even your neighbors until you agree on informal). Therapy has also always been a bit of a weird one for me, on the one hand it would be like a doctor's appointment, on the other hand the guy knows me better than most and formal doesn't seem to fit fully.

I have opted for formal, but I think my T would not be open to first names basis anyways. Since I have been wondering about this in the past, I researched this myself and at least in my language and country, it's advised for therapists to stay on a formal basis, as you said to create certain boundaries and a certain kind of distance.

I will say that I don't feel that I'm any less close to my T over this and by now I don't think much about it anymore either. The only times it comes to my mind is when my T slips up and uses the informal way. Also, although we address each other in a formal way, I'd not say that this affects how we behave otherwise, we curse, we make jokes, use irony and my T sometimes shares personal stuff. Things I'd not normally expect when being in a completely formal setting.