For a person who has a retirement visa and a re-entry stamp - if he goes overseas and comes back to Thailand, he doesn’t need to go to the immigration office within the first few days in Thailand in order to tell them that he just came back from overseas?
He just needs to report that he’s in Thailand AFTER 90 days since his arrival, which should be a regular 90 days reporting?
Back in pre-COVID days I would hang out in Thailand for 90 days (60 days Tourist Visa + 30 days extension of stay) and then bugger off to Vietnam and get another 60 days Tourist Visa from the Thai Consulate in Hanoi or Saigon. Most of the pages in my passport are stamped with Thai Immigration stamps. From year 2015 until March 2020, I would spend between 6 and 9 months of my life in Thailand. I did border bounces to Myanmar in addition to visa runs to Vietnam as well. So, I think if you are only planning on hanging out in Thailand for just 6 months - you should be fine. Having said that, beware that we are talking about Thailand, and the immigration department here seemingly endlessly tweaking and changing the rules…