Yep, I started using AGM deep cycle batteries from Batteries Plus Bulbs, but also noticed the not charging to 100% thing. I have an aftermarket stereo (Pioneer AVIC 8200NEX) that doesn't like the Forester's charging system and doesn't always start up, but it was flawless in my WRX before I moved it over when I put the stock stereo back in the WRX.
Subaru had a recall for the 3G Starlink that gave people a free LTE upgrade if done by a certain date, and if they has an active startling plan at the time it was announced. I had a Maestro RR in the WRX that let the Starlink work with 3rd party stereos, but Subaru was not able to program the Starlink without reinstalling the factory stereo (I already had Pioneer, Maestro RR, JL 5-ch amp, Rockford 750W sub amp, Focal 165FX speakers, 2x 10" subs and was too much trouble for Starlink to go stock).
I removed the Starlink and installed a Compustar instead for remote lock, unlock, alarm, GPS tracking, with phone and remote start with phone or key fob (even with a manual transmission). When we got rid of the WRX we kept the stereo but not the Compustar - wish I'd pulled that too.