Is there an alternative that focuses on local storage, which easily syncs between my devices via Dropbox etc.? Zotero comes close, but it has a weird full-text search, and the UI is not as nice as Raindrop's.
The other issue is to retroactively get local snapshots of these pages when the bookmark collection already consists of several thousand items.
Each bookmark is saved as a markdown file so syncing is straightforward. I mainly use it with the command line interface but you can also use a UI running in the browser.
ArchiveBox might be a better fit for your use case. Maybe a bit of overkill, but better to have more and not need it versus not having everything you need, right?
Bort does just that, uses browser's IndexedDB and syncs via Dropbox. But note though, that it's not similar to Pinboard etc. in the sense that it's not designed for a huge archive of rarely accessed bookmarks. Works well for something smaller, like what people commonly have as browser bookmarks.
By syncing between devices do you mean that the centralized server serves as the single source of truth, which is accessible to all your devices? Or is there storage locally on each of your devices too that is synced with this centralized server as well?
It’s way too complicated for me with a too steep learning curve, but as I understand it it’s the gold standard of “information management” and its active users adore it.
IndexedDB is probably what's meant, Local Storage (if taken literally) is a simpler and perhaps too limited a solution. There are some options that make IndexedDB easier to use, I've been using PouchDB, which works well. If syncing to Dropbox, you'd then dump the database into a JSON string/file and use the Dropbox SDKs to sync that. If syncing to a database, you can do something more efficient.
The other issue is to retroactively get local snapshots of these pages when the bookmark collection already consists of several thousand items.