Surely you need same as everyone else entering Thailand, as required to get the CoE - which won't cover your treatment as this is preexisting. Just buy the basic insurance for period you here to tick the box on entry
Next year - as in 2022. No one can advise, rules may have changed by then. Google and embassy website give you most info you need now. Non-O with no insurance not likely to be possible ongoing. Assume you will need full insurance unless you go Elite Visa route.