It took me 3 days and an online application to get an IDP in Australia, hopefully its similarly easy in Spain, it should also be cheap so just do it either way to be safe
Definitely far too unique of a case for general FB advice to be of any value to you. Seek some professional advice as suggested, and honestly if you can't afford that then i would seriously rethink your next steps
AIS is fine, i have a post paid business plan with them and my family have prepaid "tourist" sims, both have very fast speeds on 4 and 5g. I have a teenager so i would know all about it if it was slow
Bangkok and kasikorn both good for wise, many others have a cap on how much you can send per transaction. I have an account with both and they are both good. Bangkok bank app is nice and easy