you do realize standard exemptions doubled this year. You lose mortgage interest deduction also. Also business deductions, health care cost deductions have changed.
Don't assume the rules are the same as 2017. Huge differences occurred in the past year. Also as a US citizen you don't have to "prove" income but instead can just swear to it at the consulate/embassy and fill out the appropriate forms.
Call your tax accountant or see if you can get a 2018 schedule before assuming you need to withdraw savings or you will take a bigger hit if you do.
Don't know about UK but many Western countries have limits on how much cash you can physically take out of the country. You can electronically transfer whatever you want but cash is a different thing. Money laundering, narco-trafficking are reasons for cash restrictions.
airport on day you leave is easiest. You will need a passport size photo. The Thai website should have the form you fill out. That form (2 copies) passport photo, and 2 copies of your passport info page and your visa will get you thru the line faster. Add an hour to your arrival @ airport time. 3 hours instead of 2.
It is because the website requires a version of explorer that is so old you may need a floppy disc. I mean the REAL floppy disc. 5 inch diameter? Soft black plastic? Am I alone in remembering them?