@Mossop nextcloud. Requires a remote server, but has S3 backend support.
Or Borg, ditto, and mount the S3 as a file system.
Nextcloud has bonus of working on phones too
@Mossop https://github.com/s3fs-fuse/s3fs-fuse it's got the usual remote FS problem of sometimes being randomly slow, but has a local (meta data) cache to help.