So far nobody provided a good answer (if I missed it, I apologized, please do share) so I’m going to assume it’s the typical “Nobody ever get fired for buying from IBM” mindset, namely rely on what is the most popular, confirm it works well while ignoring viable alternatives IMHO, e.g NitroKey.
CopyParty https://github.com/9001/copyparty + PartyUp https://f-droid.org/packages/me.ocv.partyup/ (upload only, not sync)
Could try https://f-droid.org/packages/com.phpbg.easysync/ for sync proper.
More WebDAV Android client options https://github.com/fstanis/awesome-webdav#android but anyway my suggestion is to rely on a protocol, not an app.