Brave Sync would probably be a better fit. Brave is already built on top of Chromium, it's open source, and they recently "rebuilt [it] to be more directly compatible with the Chromium sync system" [1]. It's also E2EE and doesn't require an account.
I think what you're describing are the "Tip" buttons which can be added on a few supported sites (which can be disabled). They aren't selling anything- it's a feature which people can use if they like.
The server implementation for sync is mostly compatible with Chromium and can be used without Brave. Someone could clone https://github.com/brave/go-sync and stand up their own server. It would require some patches on top of Chromium (similar to what is done in Brave) to implement the authentication - but once that is done, all of the Chromium tools like chrome://sync-internals work just fine
The service itself is open source and can be hosted by a community. It has a different authentication scheme (ex: not Google accounts) and enforces client side encryption by default. See https://github.com/brave/go-sync for more information
The official service hosted by Brave is only intended to be used with Brave Browser
[1]: https://support.brave.com/hc/en-us/articles/360047642371-Syn...