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 don't think Keepass does syncing. I just leave the database in my Nextcloud sync folder on my desktop, and I can always pull the latest version to my phone. It'd be a bit more annoying if I routinely added new entries from my phone since Android won't let the NC app maintain bidirectional sync, but I pretty much always do it from my desktop, so the workflow is fine for me personally. If you do it on your phone, you can upload the changed version, but I think you'd have to do it manually, so not a great flow, but if it's a rarity, not too big a deal to me.