Using TLS over Tor is indeed overkill (since Tor Hidden Services are already encrypted) but if your site has a DNS-visible domain, you should be able to add a Subject Alternate Name that matches .onion in your SSL certificate – except that LetsEncrypt does not support it yet.
If indeed you have both HTTPS and HTTP+Tor, it’s a bit of a problem. Maybe instead of force_https, Discourse could use the shortcut // like for assets, or use path_only so that the full URL is determined by the host, and thus always match the calling scheme.