OSS does not mean anyone can contribute. It means anyone can view source

It's still OSS, you're allowed to fork and maintain Sqlite if you don't like the upstream merge requirements.

For instance:

https://github.com/tursodatabase/libsql

The test suite is (partially) proprietary. Good luck maintaining a fork for a DB without tests.

https://www.sqlite.org/th3.html see bottom

You can buy a license, and maintain your public fork while also letting everyone know you validated your fork with the same TH3 as the origin version. You just won't be able to show people your TH3 since I doubt you can buy a license that allows that.