Fennec is only Android, right? My issue is with the desktop version. It is also not a deal breaker, but I think it is quite a useful feature and easier to edit the search.

It is going to be interesting to see if/when they move to opencloud (from open-xchange) where they have greater control of the feature-set and bug fixes as they are both part of Heinlein Gruppe.

Either way I am happy with them so far, while also having some minor annoyances (syncing of shared task list on iOS is a bit weird, and no IMAP VTODO (combined calendar and tasks)).

I tried making my own, but generally I download them from Ankiweb. It depends on the deck how well build they are and which media they contain (audio/images/weblinks).

Anki?

FOSS and you can create your own decks or download those made by others.