That’s essentially the gist of it. 
There’s some info on the custom SQL badges in this topic - Create Triggered Custom Badge Queries
I think the query itself will be quite a simple one if you base it on something like ‘seen in the last x months’.
And a walkthrough for the specific Automation here: 'Add user to group through custom field' Automation Nope, scratch that, it’s the wrong one. I thought I’d done one for the badge version too, but now I’m not sure.
There’s a few moving parts to it, so it’d also be worth a test run to make sure everything gels together. 