Maybe I'm missing something, do you often communicate via email to your bank?

I know I do not. Pretty much never ever. That being said if I were to have to, rather than via their website or phone call, I can update that data on my bank account. I'm sure I must go through few (security) hoops to do it but I doubt it takes me more than 5min. In such case... I would put whatever email and I need to switch because I don't want to pay for that domain anymore then I would just update my contact information there.

TL;DR: I also use my own domain name for banking but in my case I'd argue it doesn't matter much.

I have selfhosted my mail on my own domain, on a server in my closet, for about 4 years. In that time I never switched over realy important things, like government etc, out of fear of missing an email and not knowing. That was the only reason not to switch for me. I've had to move a few times in a short peroid and my career started taking more of my time, so I have less time to manage the server, thus I started looking around. Now I settled on proton, I don't use any of their domains, only my own, and I've switched literally everything over, except for the recovery mail for my domain registrar login. Since proton hosts professionally, I trust them not to drop mails without telling me, and I don't realy see a reason not to switch over. If I ever want to move provider, or start selfhosting again, I'll setup the new provider, update the dns for my domain and done.

If you have some specific questions about my setup or choices, let me know, although I'm probably not able to reply in the next 12ish hours.