Ethernet built-in Port not showing in Network/Services - can't add in my MacStudio M1Max

Given your extensive troubleshooting and experience, it does seem like the onboard Ethernet hardware on your Mac Studio M1 Max could be failing, especially since it was working well for weeks and then suddenly stopped. Hardware failures in refurbished units, while less common, are definitely possible, particularly with components like the Ethernet port that can be susceptible to failure over time or due to manufacturing defects.

An Ethernet port failing is a possibility, but it is rare.

You probably should have your mac checked out, if it did fail it may be fixed under warranty.

FWIW, you could use a Thunderbolt 3 to Ethernet adapter, as you mentioned, but that would have nothing to do with Thunderbolt Bridge. The latter is a feature that allows direct connection over Thunderbolt - you could use it to connect two macs directly with a Thunderbolt cable. Not used a lot, as far as I can tell (I did try it some years ago as an alternative to Target Disk Mode to transfer a lot of data from one mac to another, and it worked, but requires some configuration).

The hardware can die, but so soon is weird. What you can do is indeed use the Thunderbolt bridge. But don’t do that yet, is your mac updated to the latest software version? If not try that first