I may be wrong and with deference to BritTim's earlier comment, if you are aiming for a visa-exempt entry, regardless of having 20k cash and on onward ticket, there's a higher probability of getting knocked back at DMK Don Meuang than trying for a visa-exempt entry at BKK Bangkok. Your history doesn't seem too questionable but the IO with a 'bad hair day' scenario seems to be more of an issue at the northern airport.
Since you have Vietnam on your radar already, what's the chance of heading over to Saigon and applying for a SETV there? There used to be a pinned 'sticky' thread with updates about the difficulty or otherwise of getting various Thai visa's at regional embassies and consulates. Looks like it's been un-pinned, maybe due to lack of current information?