I have an app called Tripit, in that I build a proposed Itinerary of my trips. I get discounted tickets and also use points, so what I do is put in the flight details of the flight I would like to be on, and if asked open the app and show that, I also add hotels, train and bus details as well. I also add legitimate stuff as well. As others say your first airline will be your hardest, but on more than one occasion I have just opened the app and shown them as well, Again at BKK airports I have never been asked for onward bookings
I have driven in with my son who has lived in BKK for 20 yrs, he travels twice a year and even he gets caught, if you have Thai Baht, they say no, need USD. If you have USD they say no need Thai baht. It's mainly customs dept and Immigration officers. Also make sure you $USD are new notes, there are millions in counterfeit floating around and even some shopping places will try to palm them off on you in change. Nice place but totally corrupt
You can get a evisa for Cambodia but make sure you have a colour print out as well, as customs will scam you, make sure your car has all its papers as well, don't cross into Cambodia at Poipet, they will try to scam you even though you have a visa, make sure they stamp you in and check the date.
Without knowing your partner or relationship with her, how long etc. I would utilise the 60day visa exempt and 30 day extension at least twice by which time you will have worked out all the shortcuts. I offer this after 52 years of marriage, where I have seen very successful relationships and many painful (and costly) breakups. There are so many customs and practices that can totally confound westerners, and you need to be aware off before you commit
Did one in Bangkok a few months ago, and when I was sitting there getting my photo taken, there was an agent next to me with 20 Chinese passports as the immi boss processed each one, the agent would hold an A4 photo of the passport holder in front of his face. Makes a farce of the system really
Even if you buy a cam visa as you cross, make sure they stamp you in as well. It cost me $us200 as they sold me the visa, decided my USD payment could be counterfeit, and demanded Thai baht, then put the visa in my passport but didn't stamp me in. When I went to cross back next day, they threatened to arrest me for being illegal. They wanted $400usd but bargained it down to $200 to get an exit stamp. Avoid Poipet
Singapore has had it for a few years. They have terminals in arrivals if you haven't done it beforehand, Thailand is complicated as they want to add money to an approved system, Australia have just about implemented a similar scheme as you get off the plane
How modern is you phone? Download an ESim from AIS or Dtac, sign up with the cheapest package they have, then login when you get there, and upgrade to the best data pack the suits you
My wife and I both have Thai Esims on our phones, both registered under her Thai ID, when in Thailand. We change the plan for the best value we need, then when leaving, change it to the lowest monthly rate, which is not much. They are good for 12 months if we forget to top up, then we just activate again on return. Both our home phones are Esims as well on our Google Pixels
My advice initially is to find out whether In fact whether you will be able to get a work visa as a "mechanic", if you are followed internationally, perhaps contact one of the racing organisations and offer to teach Thai,s your skills if they assist you. Otherwise I doubt whether will get a work visa. Only other way would be to invest capital in a new business startup. You will need to go the BOI for their rules