I've cancelled my subscription yesterday, conveniently it was about to renew at the end of the week, so I have a few days to migrate from Proton, here's what I'm thinking of, but I'd love to hear what other people are switching to
the community already made a custom server that's compatible with Bitwarden, I imagine if Bitwarden cut them off then they could make a custom client for it as well.
in fact I believe there are community made Bitwarden apps already for Linux desktops, although I never tried them because I just use the browser extension.