I always wonder how do they know the weight of the plane unless they weigh the passengers? When they charge so much for a few kilo.of bag, than why does a 40kg passenger pay the same fare as a 140kg passenger who really needs 2 seats? Some people are carrying a whole extra person around on their body but they pay the same and take up half of your seat if you are unlucky to sit next to them! Not fair really.
All the airlines are like this these days. Especially the cheap tickets. They get you with ridiculous luggage costs. Always weigh your luggage beforehand. It's always cheap to pay for extra luggage BEFORE arriving at airport. If you have to pay at rhe counter it's almost always a ridiculously high price. Last resort I use in emergency if caught out is I go and "repack" .... put as many heavy things in my pockets as possible, wear many clothes from the bag until it comes down to the limit. But 5kg over is a lot not so easy. Have a travel scale for checking weights to remove this stress from travelling.
For most people, there is no advantage or reason to have a Thai passport, especially if they have to give up their original passport. It is possible to do if your whole life is now in Thailand, you have long term job or business and can speak Thai, although the speaking Thai part perhaps is relaxed in some circumstances. After several years in Thailand I am yet to actually meet anyone in person who has done this but have come across a few online.
a bridging B visa does exactly this. My wife has been in Australia now one year, we changed to bridging visa and now can come and go as she pleases from Australia.
this is all very easy to do without paying many thousands to an agent. Just did it myself recently and was surprised how easy it was just a lot of boring paperwork.