Missing TG0B causes kernel panic, plus fluctuating battery level

Mostly filling in details here without character limits, but I think @geirandersen is probably on the money here. The 180 second timing is textbook for this sort of issue. It's just the limit the iPad has set for the amount of time it will wait to get a reading. You can likely find this information if you need reading through the panic. But low quality batteries may also suffer the same issue as your original battery.

It's also possible the pin on the logic board carrying this signal was damaged during the repair. They can be quite fragile and are easy to damage based on the way the connection needs to be separated from board. The one indicated here is the one for the NTC line if that is the issue.

Based on the schematic and boardview I have, the thermistor itself is on the battery's own board (If TG0B is actually the one on the BATT_NTC line). You can do some checking with a multimeter if you're so inclined to check for issues with this line, it can certainly be done. But it might be tricky to get a reading based on the location of the capacitor that is also on this line. It's under the shield that covers most of the logic board components. I will see how motivated I am to tear apart the iPad 7 I have to get a good picture and a known good reading once my coffee kicks in (It's not fully assembled, but the digitizer and LCD balance required to isolate the battery from the board is not my favorite).

It's might even be this isn't the issue at all. The fluctuating battery levels shouldn't be related to an NTC, that just monitors temperature. Battery levels are more likely related to the BATT_SWI line which gives "Gas Gauge" or charge level sort of data. But I'll elaborate more if necessary.