I've used https://filebrowser.org/ and it's okay. I've also Seafile, but my current setup is sftp clients (Transmit nowadays) and Syncthing if I need the files on multiple computers.
Syncthing actually doesn't provide a way to view your files online, you have to set up an instance on your local computer then have the files synced to there and then view them locally. However, on the forums I have found links to something that people seem to recommend for viewing files online, a sort of enhanced web index: https://github.com/lrsjng/h5ai.
I've used https://filebrowser.org/ and it's okay. I've also Seafile, but my current setup is sftp clients (Transmit nowadays) and Syncthing if I need the files on multiple computers.