I believe there is actually a gift tax in thailand but I do not know what the exemption amount is and it appears to be mostly voluntary. I've seen much larger cash deposits and nothing ever happens.
It depends on your destination, sometimes you don't have a choice of airlines. However, all the cheap carriers are basically the same for the most part. Try using agoda to book your hotels and use grab/bolt for taxi.
From what I've observed in thailand if you don't have money and you have criminal charges filed against you, a lot of jail time is pretty much inevitable.
I actually hate dealing with immigration and have used an agent for the past 3 years... have never stepped into an office since Covid. Some people say its a waste of money and you can do it yourself, but I dont consider the money to be a significant amount considering the time and hassle you save. Time is money.