I only have ever know one guy get a re-entry on a SETV. Went to Angkor for 3 days and from a country that could only apply for tourist visa in home country
It will be the border officials asking for the extra money for longer entry. My pal used to pay 2500 for 30 day VE and when it went to 60 days the border charge went to 3500. He used to drive himself as he had a timeline to keep to.
What treatments do you need would be a good start! EDIT: I see you added backpain and knee. You will need to find a Dr / Clinic that will detail you a long term treatment plan.
As long as you accept the risks then up to you :-)
Consequences of non-compliance:
Fine:
A fine of 2,000 Thai baht is imposed for failing to report or reporting late.
Increased fine:
If you are caught by police or immigration authorities without having reported, the fine increases to 5,000 baht.
Immigration Issues: Repeated violations can negatively impact your immigration record, making it harder to renew your visa or secure future visas and extensions.
There is a 7 day grace period but after that you may be fined. I once forgot totally and went to the office. Told one of the agents in the offices next to the Immigration Office my situation. He said OK - 500 THB and it was all sorted. That service may or may not be available where you need to report.