Thanks for your detailed explanation!
Solution #1 is already being developed as freedommarks, which has the discussed limitations, but seems to work quite nicely.
Solution #2 is being developed as floccus (by me 
). Firefox developers have recently begun working on adding support for accessing tags via the webextension API. There are other issues, but it’s a feasible approach to realize, I think.
Solution #3 wouldn’t be as simple, because browsers have different forms of storing their bookmarks, usually they lock these files while the browser is running, so you’d need to close your browser for your bookmarks to sync, and even then syncing via WebDAV wouldn’t guarantee the promises of a real bookmark sync, but would just override other changes instead of integrating them, so inevitably with this solution there would need to be new apps for all platforms.