I have a script ready for you that works just like this.
It runs continously in the background is can open a .onion URL in the Tor browser at any time interval you configure.
It doesn't use Automator (as I don't think it would be possible to acheive in automator), but it uses Python. It can be run very easily on a Mac.
Please contact me and I'd be happy to share the script with you.
Regards,
Nathan