Ugh, I've also got an Outback (2019) with a regularly dying battery. Let me know if you ever find any leads on the root cause.
The only fixes they have (as of a couple weeks ago) was to a) buy a new module (which are on back order because a number of peeps are having this problem) -- which will eventually break because they haven't reengineered it. b) disconnect the Onstar module via a circuit (my tweeters are on the same circuit, so I'd lose them) c) Open up the center console and only disconnect the Onstar module.
I opted for C (which I was charged an hour of labor for). The tech there said to check back in every few months to see if Subaru puts out a new module that fixes the issue.
Edit: The tech told me the Onstar module was a specific issue with 2019 Outback/Legacy, and that 2018 and back doesn't have that problem and in 2020 they redesigned the whole model, so the newer ones don't have that issue.
> Having that fuse out kills some speakers and the mic too, so it's a kind of annoying solution.
That's exactly why I didn't choose that option.
One thing I'll mention about the center console solution (which I'm having trouble confirming), when I was driving home I thought the sound from the speakers was much quieter. I tried zoning the music to the quiet speakers and there was little to no sound coming out. I took it back and they took the console apart and reset everything and then it "just started to work". The tech seemed as baffled as me, he was frustrated he couldn't tell me why it started working. All that's to say YMMV.
Didn’t realize this was happening until returning to a drained battery on two occasions, and then observing the effect later while doing some garden work, when I finally connected the dots. I’ve since duct taped it into the “off” position.