Just came back from KIX/TPE/SFO on CI with a 9 hours layover in TPE and went into Taipei on US passport.
All you need is to fill out the declaration form they hand out in the plane on arrival. For address in Taipei just use the flight information leaving TPE (in my case TPE/SFO) and show the boarding pass to the border control officer (I didn’t know this. The officer fill out for me). Then off I go into taipei.
They will still give you the test kit but I didn’t use them and brought them home.
The CI lounges are open with showers. I used the showers on my way to japan but when I tried to used them at night after my little sojourn into Taipei the showers were fully occupied with a long wait list. Taipei was 80 degree and humid 🙄.
Have fun.