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
Notesnook is an option that has a web-client as well as apps for pretty much any OS. They do have a free tier, but several features are hidden behind a paywall, so be aware of that.