Visa on arrival is 15 days and applies to 18 countries VOA is not a 30 days stamp which most passport holders receive
Visa exemption is where most countries are stamped in for 30 days as the title suggests this is not a 30 days visa you can extend for 30 more days at your local immigration office
Why not just apply for a TR60 tourist visa then no messing around with the 30 days extension of stay at immigration it's certainly cheaper this method from the UK not sure about CZ but would have thought similar
the OP says he can't find a flight at the right price on the 27th a 2 minute search easily found availability I do find this group very picky about what you are allowed to post
I took the liberty of checking Google flights I can see some decent fares on the 27th in fact slightly cheaper than the 31st if you want to do away with the hassle of going to an IO office and maybe better than having another covid extension if planning to return soon ( admins please feel free to edit my advise as you see fit I never know if I'm giving good advice on here ) the only thing I can think of regards to your flight comments if you already hold a ticket and they want too much to change the date but if not plenty of choice around the 12-14 k THB also Finnair is pretty decent
Hua Hin is fairly easy you can either go to the main immigration office or in the bluport shopping mall in the basement I've done it at both and in and out under half hour