Also: you can not start/stop/restart tor.service separately without leaving all other tor instances untouched.
tor.service is *not* the default service. tor.service is the collection of all service instances.
Gosh, you are right there is tor@default.service, so you are actually already doing what is being done in RPMs and there is no need to move away /etc/tor/torrc at all :) (why didn't you mention that ;).
Ok, I wrote that before actually trying to disable the default instance via systemctl disable tor@default
This does not work. I fail to disable tor@default without disabling tor.service. After a reboot it is back and running.
I noticed that this service is special since it says "static" instead of "enabled" or "disabled" on other services:
systemctl status tor@default ● tor@default.service - Anonymizing overlay network for TCP Loaded: loaded (/lib/systemd/system/tor@default.service; static)
So there is no way to disable the default instance using systemctl after all?