The extremes are a cause, but not the physical reason. Cold slows chemical reactions so in a capacity limited and high impedance battery, the voltage tanks. Heat increases regrading side reactions which damage the cells and grow impedance. Both result in the same. I’m not convinced that was the issue here. Right now temperatures are pretty mild everywhere.Batteries usually go bad when temperature starting to get colder or hotter.
6-7 year old battery owes you nothing. Keep the NOCO in the car and see how much longer it lasts while searching battery deals. It may last another 6 months and maybe it just got stressed from the constant driving. I'd go for another OEM Subaru battery and dealers often have coupons for under $100.
I think you’re on to something with the constant driving. The plates in batteries shed material as the battery is cycled. I think that long term constant driving results in a very heavy electrode, lots of movement, and thus the ability to shed material. When that material is found in the bottom of the battery case, it can facilitate a path that is an internal short, which then draws down the battery.