I'd probably buy a cheap rev ticket BKK/HKT--maybe even the SIN RT.
My family did buddy passes and ended up buying cheap connects online last minute--inside Thailand and to SIN. They were flying long hauls on DL back to States.
In a pinch there is night train BKK to HKT (themaninseat61.com) and it continues south if that's of interest. Not sure how far, but it could be an adventure!
Do a search for events and timing for each destination--full moon parties, Songkran, King's Bday, holidays, festivals etc--check them all carefully.
Hope that helps.
Doubt anyone here has a crystal ball regarding loads--we wish!

I'd get on tripadvisor BKK for advice about getting around on LCC as an option if zeds fail. I recently had to get credit for TG zeds and it took a long time--bear that in mind--buy each segment as one way--full refund if you don't use them.