@sciss It's not a turn-key solution, but I believe this could be doable on the RM.
I don't ever "auto-sync", but I can use a mix SSH and CURL to occasionally pull documents off the RM. (Recently I've been using it as a PDF reader, so I've been pushing more documents to it now). This can be done over WiFi or directly via USB (it shows up as a USB network device).
I don't use the Cloud service. In fact, I think they did some policy changes and turned off my cloud service since I didn't opt-in.
But, to answer your question, the PineNote maybe could be something (it doesn't seem as polished, but potentially more hackable)? I'd also explicitly recommend avoiding the Boox products.