I did many tourist Visas (though have not done since precovid) and never had to show anything other than the initial itinerary going into Thailand sent with the visa application form. I've heard of people getting hassled for proof of onward travel but never experienced that.
Tourist visa good for 9 months, with border runs every 3 months. Plus ed Visa as others mentioned. Do that for a few years and if you eventually get married here, could get a marriage Visa with 400k baht
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