@Steve ******
if you’re taking EVA or Asia Air (which will actually be EVA on the first leg), you should not need to pass through security IF your two flights are on one ticket. Otherwise, you’ll need to pick-up your luggage, go through customs (if need be…I don’t know their actual regs, since I’ve never left the gates) and re-check. In Thailand, you’re unlikely to need to go through customs unless you have something to declare. TSA is just like any other trip.
For your checkin, if your return trip is on the same ticket, that’s straightforward too. You just present your passport.