Both my 2018 Subaru and 2008 Totoyahaha Tundra have electrical systems completely gone awol so that leaving the battery connected results in depletion within 4-5 days. Thinking of two batteries in parallel with the ground wire connecting them routed to a switch somewhete in the dash. One battery would thus isolate to vehicle systems, the other to the starter.
Check your alternator for bad diodes first
The Outback has an electrical rap-sheet longer than my arm (collective lawsuits pending and all), haven't done anything to it yet. The Tundra was depleting the battery while parked for more than 4-5 days so I started to disconnect the battery on every shutdown, this works.
Why not just route the ground to a master disconnect switch and save yourself a bunch of time and money? No need for dual batteries on a gas engine.
I'd lose all the vehicle IT systems info, it does remain an option but the battery shelf is big enough for two slightly smaller than stock batteries. By routing the ground interconnect to a switch I'd avoid any shorting to body issues,
Should disconnect your negative post cable and put your multimeter in DC amps setting between the cable and Negative post. It may take up to 20 minutes for the computers to fall asleep, but will eventually stabilize probably at 20- 60 milliamps DC. Check yours for how much loss. Simon C
In this day and age, I do wonder why it's so hard for petrol (gas) engine ECMs to store fuel trim data with the battery disconnected.
Power where you park? Plug into battery maintainer. There are body plugs that are self ejecting when you start the vehicle. Some late model semi tractors come with factory battery disconnects.