Haven't tried it, since I only sync with my PC and cloud server. But I just checked up and Librera has an option to save progress and profiles to a custom directory, so I guess that would be the way.
These are the ones I actively use with syncthingMarkor - for quick note taking and todo'sLibrera reader - ebook readerSeeneva - smart comic readerVoice - Audiobook playerSimple music playerKeeepassDx - also
Haven't tried it, since I only sync with my PC and cloud server. But I just checked up and Librera has an option to save progress and profiles to a custom directory, so I guess that would be the way.