iPad not charging properly: battery or other IC broken?

My iPad is not charging (properly) anymore and I am trying to figure out (before opening the device) if a battery replacement will likely solve the issue or if there's a high chance that some other internal IC is toast. I know this is a bit speculative but I'm interested if the error symptoms give any indication at all.

Current condition: When I plug in the USB cable, the screen stays black or shows the empty battery symbol (sometimes with/sometimes without the cable icon). After a while it tries to boot, and often gets to the lockscreen. Then it freezes and shuts down.

Troubleshooting attempts:

• different power adaptors (both USB-C and USB-A output)
• different cables
• charging via smart connector (magic keyboard with power passthrough; after it reached the lockscreen I could see "not charging" for a brief moment)
• cleaning USB-C port using isopropyl alcohol
• let it connected to power for a few hours and try to turn on again (doesn't collect charge since it automatically tries to power on every few minutes)

Observations:

• I only have a lousy USB-C cable for power readings. It would often show ~2W for a while. When the iPad then tries to boot, it drops to 0W.
• A few weeks before, it became finicky to charge the iPad. I thought I had a loose contact because certain cables and positions would still work. I cleaned the port using isopropyl alcohol and it seemed to have solved the issue for a few weeks - until now. But somehow I don't believe the port is the culprit anymore because the smart connector has the same behavior.
• battery seemed a bit weak for the last year or so (but the device is also 6 years old): sometimes I'd leave the iPad for a day or two and it would run out of battery.

Happy to provide additional information if that helps to narrow it down.