Rclone works but is still not very good, either it doesn't allow a lot of the things you'd expect from a filesystem or it'll just consume tremendous amount of disk space while it slowly transfers the files to the remote host (even on a really fast link).

As it happens, I've been deep in the with rclone, webdav, and Nextcloud over the last few days.

It works. It's slow when synching a ton of small files but besides that I haven't run into any unexpected problems.

If I did it again I'd probably use the Nextcloud client in API mode. But this was originally supposed to be a small project... Oh well.