Can my partner successfully renew a METV in Thailand after it expires and what are the potential immigration issues?

Perhaps it’s a better Idea to get a student visa and study thai or muay thai for a year and then get the DTV. But only do that if you really will attend classes.

Historically, consular offices have been hesitant to issue back to back METVs and even if they do get another, immigration will become increasingly less likely to allow them in without scrutiny.

If they do get another METV I’d be sure they have 20k baht (can be in other currencies as well) in cash and physically on them, a confirmed flight out at or before the 60 day mark (the 30 day extension is technically not guaranteed) and proof of accommodation (ideally a hotel booking). With those they should be able to satisfy the sort of easy things immigration go to when they want to deny entry but if an immigration officer really doesn’t want to let someone in they’ll find something else.

If you weren't anonymous I should give you advice

Back to back METV not a good idea

You can get 9 months out out of the 6 month METV with a last minute border bounce

There is no limit on how many you can buy back to back…but as a rule of thumb always remember there is a disconnect between the Embassy/Consulate and immigration

You will have to travel to your passport(s) country or a country you have permanent residence in to apply for the visa

Good Luck

He would first of all have to travel back to his home country to apply. Then the embassy in that country might or might not issue him with another METV. Then immigration might or might not deny him entry. I've seen reports of people trying to squeeze an additional 60 days out of an METV just before it expires being denied entry, so I would suggest his chances are not good. Probably best to leave Thailand until he has saved enough to apply for his desired visa. It's really not possible to live in Thailand any more on tourist/visa exempt entries