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
AFAIK the concept behind keepassxc is that it's an encrypted .kdbx file that you sync yourself among your devices by any means you can. There are official apps for Windows/Linux/Mac, browser extensions for Firefox/Chrome/Chromium/Vivaldi, there's integration with ssh-agent, and 3rd-party apps for Android and iOS.