What is causing the battery to die?

There could be a battery drain but if the 20's are like past years the battery is simply too small. A heavy duty, large capacity battery may solve your problems.

Not too small, FOR, just poor quality Panasonics.

So what is the appropriate size battery for my car?

I don't think there is a look up available for the 2020 cars but I use an Optima Series 35 battery in my 2015 Forester.

The biggest that will fit. I don't know if the 2020 Leg/OB reverted to the polarity geometry used 1990-2014, or the "newer" reversed polarity of 2015-2019. Just look under the hood at your OE battery and match it. Pretty easy.