Nice article!
An interesting tool that matches the requirements mentioned in the article is Evolu[0]
It's a sync engine with e2e encryption based on SQLite.
The local-first landscape is quite wide now, and there is probably a solution ready for all kind of needs[1]
Building a sync engine can be a nice learning experience, but for production software it's better to pick something that has already faced and resolved all the weird edge cases you get when building a sync engine and persistent storage on the browser.