Just returned from a trip to Taiwan, and a few notes:

On the Taiwan Mobile website, there is the $180NT deal for 1GB. I didn't know about this and signed up for the unlimited data for $500NT (including $50NT credit), or something like that. No problems with service (other than in tunnels and in the mountains), but when obtaining the SIM, was given a really hard time about the ID.

The store I went to was in Taipei near the Luzhou MRT station. They said both IDs had to be a national ID or Taiwanese, as in a passport + Taiwan ID or Passport + international driver's license. State driver's license did not work because it wasn't national. I wonder if my Global Entry ID card would have worked.

I got it registered via a friend, but then my brother was able to get it registered using just his passport and driver's license under an exception, after they saw all the trouble we were going through. This was all in Mandarin and I had Taiwanese friends helping. I don't know what your chances are if you don't speak Mandarin. Airport might be different-- when I arrived all counters were closed except for the FarEasTone counter, and I hadn't heard of them before so I passed. In retrospect, they probably would have been fine. Cost at airport for the various unlimited data plans from each company are all similar around $500NT.