That's funny because more than half the people in my Thai class was from countries nearby. Myanmar, China, Vietnam, South Korea, very few people from the West.
I use Google fi. after 3 months they cut off my internet but calls and texts ok. free calls via wifi if you're on it. I'm not recommending it, just telling what I use.
I also have a AIS sim and number here. Make sure, if you are staying in Thailand, that you get a proper sim and number. many are tourist numbers and can be restricted.
I think John gave you The best answer. truthfully there is no easy answer. if you're wealthy Thailand will welcome you. if you're not wealthy Thailand will still welcome you but not for a long time. 3 to 6 months. there are multiple options for visas and they are changing constantly. I recommend trying to get your Visa in your country before leaving but that will mean you will have to have Thai health insurance but with your age it will not be too expensive. I don't have a good answer on whether you could use one bank account you would think that it would be okay but you never know.
of course your other choice is to use an agent. that is the simple way that will get you a Visa.
agree. 1 stop only. first time was from LA to BKK via hong Kong... never again lol!. next was from Philly to Boston, then Istanbul to bkk... both times less than $1500... and last time one way only with bus ticket to Cambodia if needed... works for me. good luck.