If when you did your first Extension of Stay the requirement was 400k and you’ve done annual Extensions without a break every year since, then the old rule still applies to you. Won’t be many people left that this applies to though.
When you enter Visa Exempt you get 30 days. You can then extend once for another 30 days. However, under special Covid rules you can then apply for a ‘Covid extension’ to stay longer.
They may stop giving this extension but you can currently apply.
They are currently still giving Covid extensions, though they’ve become more difficult. You need to go to your nearest Immigration Office with the necessary paperwork and apply. That way you don’t need to leave the country.
You can. Companies used to offer ‘border runs’ where you’d jump in a minivan and they’d take you to the nearest border and back again. More complicated due to Covid. Land borders only just opening again and you need a Thailand Pass.
If you do it too many times you can be flagged by immigration.
You can stay in Thailand permanently by getting an Extension of Stay each year. You never need another visa. If you want to leave Thailand and return you get a Re-entry Permit. You can do this as many times as you like.
if he plans on doing it all himself then it’s unlikely he’d manage it - though it depends on the Immigration Office. Last person I know that converted from TR needed 8 visits to the office as they kept asking for more documentation.
You’ll need to get a Tourist Visa (TR) before entering then, yes, that can be converted to either a Non O for Retirement or a Non O based on Marriage within Thailand.