2005 Prius diags, acceleration issues then parked for a year

I'm trying to help a friend. I've fixed my own cars for decades but have no Prius experience. I do understand electricity and safety. I don't necessarily expect to be able to fix this (maybe) but would appreciate guidance in troubleshooting so we can figure out which direction to go.

History and symptoms...

2005 Prius, 225k miles. A year ago it was parked for 2 months then only driven occasionally. One day it wouldn't start. Jumping it worked but after driving about 15 min it would go silent and only accelerate to about 10mph. Eventually it would accelerate normally again but each time was different, sometimes after a minute, sometimes several. After a bit more use it wouldn't accelerate unless you got speed (such as downhill). Then it stopped being able to start on its own, needing a jump every time. The 12V battery was replaced but it didn't help. It's now sat for maybe 8-9 months.

This is where I got involved, here's what I did...

The car was unresponsive until a 12v jump pack was hooked. I pulled codes P3000 and P3030. I ran Dr. Prius even though I haven't yet read how to use it or if it works with dead batteries. The bars on the left half were fluctuating, some red, some going between red and not red (colorblind). Screenshot attached.

I understand P3030 could be damaged connections in the battery bus harness (battery monitoring system?) but also this car is 20 years old, how long do these battery packs live?

My plan is to bring a plug in charger and get the 12v battery charged fully (if it's not ruined) before trying to collect more information, hopefully with guidance from this forum. I plan to also check connector BM1 in the rear right which I understand to be a common corrosion point.

Advice is welcome. Thanks.