This is a bit tricky! As we need a USB-C power meter to measure the ports power to see if the charging logic is working correctly. The trackpad haptic feedback doesn’t pull that much power and is on its own circuit from the rest of the system so don’t read to much into that.
Given the age of the system your Battery is likely on its last legs and will need replacing soon the fact you disconnected it from the AC may have broken the camels back sadly.
Don’t forget this series had the Stage light and the butterfly keys issues. Apple was not kind to the people who bought this systems and made it quite hard to fix. Neither of these are easy fixes.
So not to be a downer I’m not sure if this system is worth the investment. So before you go to far see what you can get for a used 2018/19 system with equal if not better specs and options (RAM & Storage)
I don’t like telling people this news, it’s simply one of the worst systems Apple has produced. I still use a 2015 15” MacBook Pro as I just didn’t like what Apple did. I often get people just do simple stuff to get one if they can get a good price. Many people are holding tight to them until Apple wakes up!