You might check out Nextcloud. A hosted version of course, since self-hosting could not be called a Dropbox replacement. Hetzner + Nextcloud [1] replaced Dropbox for me last year.
Apple forces the use of this API if you want files to be downloaded on-demand to save disk space. Nextcloud seems to support this use-case on macOS only experimentally [0]. In the light of these changes, they might decide they don’t want this feature at all on macOS and sync everything the old way. (I don’t know what their plans are, just speculating.)
I actually tried this and wanted to like it but the problem is that they only have NextCloud-specific hosting in Germany at this time (unless that changed recently). That’s good if GDPR is a real concern for you, but it just kills the speed if you’re on the other side of the ocean.
I worked NextCloud into my setup locally though. I don’t love its syncing capabilities, but you can get around those if you set up SyncThing folders as external storage. That way you get the benefits of NextCloud (the customizable interface) while getting the fast syncing speeds of SyncThing.
[1]: https://www.hetzner.com/storage/storage-share