an exemption stamp is per definition not a visa. It's a granted entry without visa, aimed at tourists. But you do you. Some people can use those lots of times. Even back to back. Other people will get pulled aside and questioned only after a few times. All will eventually get denied entry if they keep doing that.
that depends what kind of ed visa. If from some shady language school performing as a visa factory, then yes. Immigration knows what's up with that and will possibly deny changing to other visa types after.
Having had an ed visa from a respected uni? No problem whatsoever.
If doing it at maximum per semester, it's closer to 30K per half year in school fees. It's pay per credit. Theres a minimum amount of credits you need for immigration to accept it and provide a visa, and a maximum of what is allowed per semester. I think minimum is 9 credits, and maximum is 21.
Sign up at a known language school learning Thai. That'll give you a year's worth of visa.
Or sign up to study at a university. Theres some cheap bachelor degrees around that will both give you a visa for up to 6 years, and land you a degree if you finish. Cost is somewhere around 15K baht per half year in school fees, to satisfy the minimum requirements for getting a visa from immigration.