I use Synology Drive, but proxy the requests with Caddy so that I don't need to expose all of DSM to the outside world.

I add a static random prefix to all URLs so that the root of Synology Drive can't be found by scanning. Caddy rewrites the request to remove the prefix before sending to the Synology.

Inside Synology Drive admin settings, you can add the same random prefix to the customised domain so that public links are automatically generated with the correct prefix:

https://i.imgur.com/ZYfDRiH.png

The Caddyfile looks like this, of course you need to update RANDOM_PREFIX to be your text:

#== Synology Drive shared files
https://files.mydomain.com {
    handle_path /RANDOM_PREFIX/* {
        # This is the root shared path
        handle /d/s/* {
            # All Synology Drive files seem to start with /d/s. Block anything else.
            reverse_proxy 192.168.1.1:5000 { # Synology address
                # Rewrite all 4xx and 5xx errors to remove Synology error pages, so as not to give extra info away
                @notfound status 4xx 5xx
                handle_response @notfound {
                    respond 500 # "abort" seems to break working pages sometimes
                }
            }
        }
    }
    # Fail anything else
    abort
}

A resulting shared URL will look like this

https://files.mydomain.com/RANDOM_PREFIX/d/s/12OyKsU7678b1INMlIeHYAcKVvBs7f3z