I upgraded the battery from factory to a larger capacity, hybrid deep cycle, and that has increased idle discharge time by a couple of additional weeks.
So a low quality OEM battery compounded the issue, but either way the electrical system leaves a lot to be desired.
The guy there didn't even realize they sold a correctly sized one for Subaru Outbacks, because they're typically for e.g. larger pickups or similar.
For $50 more, you're getting a much nicer battery: AGM internal structure, higher reserve capacity, higher CCA, better durability, some deep cycle properties (i.e. it can get drained down lower without damage), and so on.
I mean it is 10 lbs heavier, while fitting into the same physical space, that's the AGM structure within the battery. The new battery has been great, but the alternator is still not kicking on until freeway driving.