I want proton Mail to always open in a folder specified by me.

This could also be "latest opened folder".

Right now I always see the inbox, which ideally is pretty empty in my setup, since I have a ton of subfolders and filters.

This means that I have to go to the "All mail" section - or browse many subfolders - to see discover new mails.

I'd personally prefer to always see "All mail" when opening Proton on the web or in an app, but another solution could simply be that Proton opened in the folder that was active when it was closed.