MacBook Pro, inactivating/disabling USB peripherals

Is the charging cable connected directly to the laptop? Have you tried using a different USB-C port especially on the other side of the laptop if you have the four port model?

It is possible an I/O Board (aka USB-C port) is bad or even a bad Logic Board. If your external devices are working fine using the laptop's battery and you only get the error immediately after connecting the charger directly to the laptop, then it likely means one of these two parts are bad (I/O Board, or Logic Board). Unfortunately Apple techs are not trained to troubleshoot odd issues involving third party accessories if their service Apple Diagnostics are not reporting any problems. If I were the Apple tech, I would be physically examining the contacts on the USB-C ports on the laptop to make sure they don't have liquid damage or burn marks on them. It is extremely hard to be sure the contacts are clean just by looking into the ports since I've thought the contacts were good until I removed the I/O Board for better examination. The Apple tech should be trying new I/O Boards in the shop to test whether they are the problem or whether the Logic Board is to blame (assuming the issues does not occur with another identical laptop using the same charger and peripherals).

Having drives disconnect before being properly unmounted can corrupt the file system on the drive. If the drive is using the new Apple APFS file system, then there are no third party utilities available to repair a damaged APFS file system if First Aid is unable to repair it.

Unfortunately the two port non-touchbar model does not have a separate I/O Board, so the USB-C ports are directly attached to the Logic Board.

You may want to try rotating the connector on the USB-C charging cable 180 degrees upside down to see if that makes a difference. If the USB-C port is only half bad, then this may allow things to work. However, it most likely means you have a bad Logic Board. If the contacts have liquid or corrosion on them, then sometimes it is possible to clean them up, but it is doubtful Apple would try this although an AASP might. Assuming the Logic Board is Ok, maybe you can see if using a larger 87W charger will work.

If you take your laptop to Apple to be examined, then I suggest taking your charger & charging cable and perhaps a drive(s) or devices which will cause the problem so you can reproduce it for the Apple tech. It really helps if you know how to reproduce the issue fairly regularly.