If you go the STV visa, then you need to leave the country. You cannot apply for another visa in Thailand while on the STV. Now that you booked your return flight, you cannot use TR visa, since you need a onward/exiting flight 59 days. The STV visa is stupid and costly as
mentioned. What you should have done is the 60 day TR visa. Extended 30 days and can still keep your 89 day flight by using websites that do fake onward flights.
contact schools directly, don't use agents. Duke, ALA are 2 schools I recommend. You can't get away with the old school Ed visa any longer, you'll actually need to attend 4-5 days a week.
I'm doing the ED visa. Its 8 months currently and yes there is some commitment to learn Thai, but it gets you 8 months (10 if you do the 60 day tourist visa before). Not all people want to commit to learning Thai, but it's pretty helpful when you stay long term in Thailand. These are just my opinions.