I have used all 3 carriers over the years, the first when I came to Taiwan was Chunghua. I didn’t even have an arc but the nice lady at the corporate locations hooked me up without a deposit. I’ve heard some locations insists foreigners pay a $2900 deposit as I remember.
Signed up Fareastone as well back many years ago as a backup phone, they wanted the $2900 deposit AND they freaked out that my arc was only 11months and 27 days long, they insisted it had to be a full year. Sent a complaint email to them and they immediately offered to set me up without deposit.
Changed my chunghua line to Taiwan Mobile because CHT charges too much, had already been in Taiwan for 5 years with perfect payment record, Taiwan Mobile insisted on a deposit of $2900 . After speaking to a manager they again waived it but they really did not want to.
So in total , expect to put up a deposit or a fight. Once set up all 3 carriers are good but interestingly as I know only Fareastone has special plans they only let locals get which sucks.
Taiwan star I have not been with but they are the worst as you can’t put a deposit, you need a local to cosign to set up.