Still halfway through reading, but what you've made can unlock a lot of use cases.

> I tried SQLite first, but its extension API is limited and write performance with custom storage was painfully slow

For many use cases, write performance does not matter much. Other than the initial import, in many cases we don't change text that fast. But the simpler logistics of having a sqlite database, with the dual (git+SQL) access to text is huge.

That said, for the specific use case I have in mind, postgres is perfectly fine