So today the bottom module and battery arrived. I swapped out the bottom module and both the old and the new battery charge at 2 amperes again! So problem solved.
I blame corrosion on the outer contacts of the charging port, as the insides seemed to be really dry. You could clearly see the corrosion when comparing the old and the new module side by side.
I’m keeping the battery as a backup for trips and such.
I indeed tried that and also installed a battery app that lets me know at all times how much power is drawn or charged. Since it communicated normally, I was pretty sure the charging chip in the core module was unharmed.
Btw I love that you get a return label to send in your replaced parts, that’s outstanding service!
Thanks for everyone here giving me advice and good look to the other person with the same problem.