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
I recently found https://www.aliasvault.com/. It is still in early-ish development but the 1.0 release including a security audit is planned for end of this or beginning of next year. The main developer is also very active on https://www.privacyguides.org/ and listens to community feedback.
Cant wait for it to mature a bit. Right now my biggest gripe is UI/UX related.