FLight and Visa question coming into Taiwan

I’m going to be going to Taiwan in the near future with my girlfriend who is taiwanese now a couple of questions first of all I’m uk national with a UK passport I am I correct in thinking I can get a visa on arrival for 90 days?

The second question and the one I’m most unsure about is I’m flying into taiwan from Thailand and its only a one way ticket as I have no idea how long i’m going to be in Taiwan for. Is this going to be a issue with immigration/Air Asia ?

If it is going to be a problem , which is the cheapest way to get a flight ticket to keep Immigration/Air Asia happy?

I have searched for this but all the answers seem a couple of years old and I know how quick these things can change

many thanks in advance