For PG...XXX-BKK-NNN
Chiang Mai, Chiang Rai, Krabi, Phuket, Samui, Trat and Udon Thani
https://bangkokair.com/eng/pages/view/check-in-through
There was some PR for UTH and giving FD the same service via DMK. Not sure if that is seamless or available?
But these Domestic-BKK-Int'l (C-Customs, I-Inspection/Immigration, Q-Quarantine) procedures aren't the same as what the OP was having problems with, as far as I can tell?
Posts/threads in the TG forum here indicate both success and failure re: Thai Smile and CIQ procedures via BKK.