I had tried Zotero before but I stuck with Mendeley for a long time. I ended up using Zotero a couple years ago to share some references with my advisor and since then I've grown to love it. One of the nice things about it being based on Firefox is that there's a lot of extensions written for it (which IMO, are not well-advertised).
The API is also great and I've been using it to automatically sync PDFs of papers I want to read to my reMarkable tablet.
One thing I'll add that has happened since the post was made is that the reMarkable team has been pretty good at churning out updates. The last firmware update adds table of contents and text search for PDFs and its incredibly fast.
My two biggest annoyances are currently:
1) No web app, so the only way to export PDFs is via the desktop app which is only available for Windows and Mac. The Android app is also pretty terrible. I think not starting with a web interface was a bad decision.
2) The PDFs exported from the desktop app are huge (a PDF that started out ~400KB ended up >100MB after export). Support tells me they're working on this.
The API is also great and I've been using it to automatically sync PDFs of papers I want to read to my reMarkable tablet.
https://michael.mior.ca/blog/zotero-remarkable-sync/