The iPad Mini 2 is 9 years old. The battery would be suspect, even if it was 97%, it should drop to 20% pretty fast. Does it boot when plugged in? Or does it need the heat?
I would guess there is a loose wire in the power delivery, possibly on the battery or somewhere between it an the board. The heat is causing enough expansion to make the connection. I guess even plugged in, the heat is needed.
3utools looks sketchy. I would not recommend anyone use it for anything, nor would I trust what it tells you.
I am assuming you want the entire iPad Mini to the iPad 8th gen. I assume both devices are under the Apple ID.
I would say, plug the iPad Mini 2nd Gen into power, heat it with the hair dryer, tell it to sync wirelessly with the iTunes it is associated with, and keep the heat on the iPad Mini until the backup is complete. You may need to connect the iPad Mini to the iTunes computer to get the backup, still using heat to keep it running.
Then plug the iPad 8th gen into the same iTunes and restore the iPad 8th gen, with the iPad Mini backup.
Erase (while heating) and recycle the iPad Mini.