If you do land border runs with a visa run company or drive yourself to the borders they use and pay the border fee then you can live in Thailand non stop. You will have no issues. But if you fly in and out there is a high chance of having troubles.
I lived in Thailand for the past 3 years by doing land border runs and never had any troubles
The QR code is just a short cut link to their bank account. You can just ask for the account number and then transfer using wise or one of those companies with your home country account
I don't know any agencies but if you want to do it yourself and save a lot of money it's not hard to do. I paid for a 6 month training package with Phuket King for 15,000 baht and then applied. They have all the correct documents.
for once I will agree with you. 27k is a fair price and that gym Fitness Hero in Patong he got the price from is actually a really nice gym. Very clean and modern with good equipment, I have trained there before.