I use businessclass.com

Overstays can and seem more likely to impact any future long term visa requests. Avoid if possible.

Learning to speak is much easier than understanding spoken Thai. I recommend learning the Thai alphabet and inflections used as early as possible. It’s taken me about 1 year to be able to have full on conversations with Thai friends… the more you learn the faster it comes to you. Now working on my Issan!

For UK it will be very difficult. You will need to show lots of money in your UK bank account to cover all her costs. If you own UK property show details. Have detailed documentation and timeline including photo journal of all the time you have spent with your girlfriend. Even with all this you probable have less than 1% chance you will get a visa for her. For UK you will normally need GF to have job for reputable registered Thai business, to own property, to have savings.

Just book the return flight for 6 months in the future. Also, if it's business class you can always move the date again at no cost.... you never know

Recommended to do 3 days before travel.

Any site asking for payment for the TDAC is being processed via an agent. No need to pay.

maybe a good option.... As you state you only need this insurance for your visa therefore you can increase the excess to significantly reduce the cost.

Taking the fine WILL impact your ability to get long term visas in the future.

You need the money!