Yes, I was told you can take Bhikkhu ordination, remain in robe and stay in a monastery as a monk under retirement visa but I don't think you can show up to Chaeng Wattana or Chamchuree Square in robe asking for Non-Imm B extension. 😅

And the institution that has a final say in approving extension or reapplication? is the National Office of Buddhism (สำนักพุท) and without their endorsement/recommendation letter immigration bureau will not issue fresh new visa or extension.

If you're from non-Buddhist countries (all western countries) your Non-Imm R can be extended for up to 10 yrs (5 yrs if you're from Buddhist countries). That's not always observed and sometimes they issue recommendation letter toward 6th extention (or 11th for farang monks).

I know western monks who remained in robes in Thailand for 30 years or longer and at least one American monk who received a PR despite monastics not eligible for PR application other than under "special circumstances" category and two farang monks who were granted citizenships by HM the King but I heard reapplication will become more and more difficult to justify and convince National Office of Buddhism as years go by and beyond that I know nothing.

Wat Pah Nanachat, aka Int'l Forest Monastery in Ubon has a Thai lay supporter/secretary who is very versed in those matters but he has his hands tied up with so many foreign monks at his monastery and I'm not allowed to consult him unless I go visit him in person at the monastery in Ubon.