You have done a good job of troubleshooting. I agree you appear to have a hardware issue with the laptop which will need to be repaired. Unfortunately it is hard to say whether it will be a less expensive I/O Board repair or an expensive Logic Board repair. Have Apple or an Apple Authorized Service Provider examine the laptop to provide you with a repair estimate.
FYI, while supporting my organization's Macs....many times I do find the USB-C ports become damaged from liquid damage since it only takes a drop of water inside the port for it to start to corrode the contacts. Unfortunately looking at the contacts through the port opening won't always allow you to see the corrosion or even just oxidation that may occur on the contacts since there is so little room to view the contacts by peering into the small opening. I have never encountered a bad I/O Board resulting from a "natural" failure.