Both auto-forwarding and auto-reply are paid features, which makes cancelling & switching much more difficult. Gmail is a breeze comparatively. I highly recommend against using their addresses (e.g. protonmail.com, proton.me, pm.me)
Email forwarding is available for everyone with a paid Proton Mail plan.
(source)
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.