I used KODI for a while but switched to "just" minidlna/ReadyMedia for a lighter setup. I know that VLC can resume videos even over DLNA so I'm wondering if it can abstracted away, e.g having a VLC script that checked, maybe based on filename, if the opening file has already been played from another location and if so, resume from there.

I see that in ~/.config/vlc/vlc-qt-interface.conf there is a [RecentsMRL] section with list (unfortunately local only I believe) and times (in seconds I imagine). One could imagine to scp that file to the NAS using e.g inotify (after writes) then merge it (not replace) on that section only, then a shortcut that before starting VLC get the merged file and does the same. That should allow for resuming across devices.