Thank you for detailed report!
Couple of things regarding the commute,
you said Bangkok Airways is expensive airline, it is a low cost, budget carrier, it's impossible fly to PP and Siem Reap for less. Bangkok Air match prices on most routes to Air Asia fares, they are direct competitors. Bangkok Air usual BKK to PP rate is 1400 baht one way. How can you call this expensive? Air Asia matches that price... But,
Bangkok Air flies Suwarna, and for most people Suwarna is much preferable to DMK, Air Asia flies DMK only.
For traveller from Pattaya, getting to DMK is painfully time consuming, you won't find Pattaya - DMK taxi for less than 2000 baht, while Suwarna is 1000 baht any time. There is no direct bus line. There is a minivan tho, but if you are unlucky, the van is full, and since you have not much time left, your only option then is 2000 b taxi, I've been quoted 2500 as well.
If you really want to take a bus from Pattaya to DMK, you go to North Rd bus terminal, catch a bus to Ekkamai, or anywhere on BTS line, then Skytrain to Mo Chit, and from there A1 bus to DMK. If your flight from DMK is, say, at 20.00, you must leave Pattaya at least 14.00, I'd leave even earlier, BTS can be crowded at rush hour, A1 bus may get stuck in Bangkok traffic. Pattaya to Suwarna is 1.5 hours on bus or taxi.
Coming back with Bangkok Air you are landing at Suwarna and you can take super cheap 120 b bus to Pattaya, I find bus seats much more comfy than taxi seats, taxi usually Toyota Camry.
DMK offers no bus to Pattaya, DMK does have a shuttle every 30 min to Suwarna, but it can take 40 minutes, if there's traffic.
And then there's the DMK itself, smelly rundown shithole.
Flying to Cambodia, Bangkok Air is best choice, but even better option, if possible, avoid Cambodia.