[danielspace23]

The PKM I've been using lately, SiYuan, does exactly that, and I think their business model isn't bad: the client is fully FOSS, there are some client-side paid features with a one-time subscription (WebDAV/S3 sync "bring your own server") and some server-side paid features with a more expensive recurring subscription (cloud space provided by them).

I don't particularly like client-side paid features, but:

- The client is fully FOSS, you can just patch the license check out. In fact, there are some forks on GitHub that do just that and provide binaries, and the authors don't seem to care, they even acknowledged them on Twitter (https://x.com/b3logos/status/1928366043094724937).

- There are plugins to sync without a paid plan

This works out quite well for them: if you choose a fork or a sync plugin, you don't get the same support that paying users do, so many users still end up buying a license. But you don't need to, which makes the whole thing not user-hostile.

I have bought a one-time license myself, and I'm very happy that I'm supporting the development of a FOSS project.