There are many options.

I have been looking into this for other reasons: https://codeberg.org/shroff/phylum but it might do the trick for you.

Or this one: https://www.filestash.app/

You could install Nextcloud on the VPS, configure Nextcloud to store things in S3, and use Nextcloud's apps on the clients. And have Nextcloud provide webdav services and use your client OSs native webdav tooling.

RClone maybe? https://rclone.org/

There are some open source rclone android apps like RCX, and on windows/linux you can just use rclone mount to use it like a normal filesystem.

You can use S3FS to mount an s3 bucket as a folder and use it like a normal FS, however if you're trying to modify files on different systems at the same time you can run into caching issues

If your VPS’ S3 supports tokenized urls for object management, I’ve considered setting up a homebrew Dropbox-like solution using rsync and cron.