Bangkok Airways and THAI Smile are good, and both operate from BKK (vs some LCC at DMK). Never tried VietJet. Bangkok Aiirways provides a basic lounge service free to all pax, which is better than nothing

If in J on CX, you get a fast tack entry card which provides some more comfort around arrival processing time. 6 hours is unnecessarily long to hang out at the domestic section of the terminal. I might prefer the 14:30 PG flight to give a better buffer, but the 13:15 TG flight should be fine.