Reading through it, this is *completely* impossible for them to implement effectively. (So many countries, so many tax systems, figuring out how much tax was already paid abroad by each individual, etc. etc.) Furthermore, even if they are somehow successful then this will cause people (Thais and foreigners alike) to bring as little money into Thailand as possible. The revenue peeps will be scratching their heads and wondering if this is actually what they want? Less money into Thailand? Really? Don't you guys want more money into Thailand, not less?
Bus or taxi to Suvarnabhumi, then flight to Chiang Mai. Viertjet Air is often affordable. Or, taxi or van to Utapao and then fly from there on Lion Air. But just one flight a day from Utapao.
If going to Suvarnabhumi, consider the Bell Travel bus; they'll pick you up at your hotel (or anywhere in town) and then take you to the big bus to the airport. Big bus takes about an hour and 45 minutes.
Just book; does it really matter if you get an additional 30 days for free if it changes to 60 days? Even if it doesn't change and everything stays the same then you just do a tourist visa. You're acting like the world is about to change in some meaningful way; it's not. It's just a trip to Thailand, and you'll have plenty visa options.
They don't need to check your visa. This is on the way out, remember? All it needs to know is the date you're allowed to stay until (which is entered by the immigration official when you come in or extend your visa) and today's date. Then everyone who's not on overstay can walk straight through.. ... ... in theory... in practice I bet there will still be someone putting a departure stamp in your passport.. they do love their stamps. 😃
Or you could have been appreciative of all the positive, helpful replies you got that fully answered your question. Instead you choose to focus on a couple negative replies. Welcome to Facebook.
Immigration lady clearly wants to marry you. 😉 But seriously, really a lot of people have multiple trips to Thailand. You just need to make sure you look wealthy, or look like a sex tourist. Both seem to be preferred types. Don't look like a young hippie. And make sure you meet ALL the rules, including having really a lot of cash on you, have an onward ticket, etc. And relax a little; you seem serious.