Nextcloud

Ipfs or possibly arweave

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.

PeerGOS is the best there is. Encrypted, self-hostable (FOSS of course), federated, peer-to-peer features, and its own app ecosystem. I consider it the crown jewel of federated technologies along with XMPP, Delta Chat, and Peertube

Nextcloud's encryption... good luck even getting it to work lol. Might as well automatically zip and password lock stuff instead oof

Please support the devs I have found nothing like it anywhere!! (Currently their own offerings are inefficient compared to other datacenters but I buy it basically just to keep the lights on.)

Seems like Seafile has an app on FDroid: https://f-droid.org/en/packages/com.seafile.seadroid2/

Syncthing

Selfhost Nextcloud.

I'm using an anon Dropbox account service through Cryptomator. Cryotomator is a really strong encryption cloud storage app. It just uses your Dropbox account to upload it. Cryotomator might be open-source but dropbox definitely isn't. So sorta?

Seafile

Self host filebrowser quantum! Small, light and easy to configure.

It is currently in beta, but it is on the cusp of a stable release.

I selfhost NextCloud, and I haven't had any problems at all... and it's between windows, android and linux...