Sharing my return-to-Thailand-with-a-passport-full-of-covid-extensions experience, for those to whom it may help...
After 2 full years of living in Thailand on the extension program I was warned that, regardless of how long I had been out of country (5 months), trying to return to Thailand that I should expect to be hassled at least and refused entry at worst. Turns out that re-entering through BKK was not difficult at all. But there were 2 particulars that made it frictionless;
-An issued receipt that showed where I was going to be living for the duration of stay
-Proof of onward travel. (this one was most certainly pivotal, since not only did they ask this at BKK immigration but also in Phnom Penh at the airport check-in counter before I even left. I'm assuming that enough people were getting turned around at BKK arrival for them to start asking this question to travelers before they even departed. But this is just an assumption).
At the additional recommendation of the awesome Tod Daniels I also bought +20k B cash, but this was never asked for. This might be non-news for most now, but absolutely nothing covid related (test, vaccination) was even brought up at any point from departure to arrival. What a turnaround from even just a few months ago.
Cheers...