The serial numbers are on the back of the devices so it’s nice to be able to correlate them with the network name. As I mentioned now we prefix them with a few letters to avoid being misinterpreted.
Base64 needs 64 unique characters, but domains names don't have 64 unique characters. Also base64 could result in pure numbers as its output, leading to the exact same problem.
Do you actually use serial numbers anywhere else?
Why not generate a random name instead?