go to BKK instead and then get a cheap internal flight to Phuket, you will probably find it works out a lot cheaper. Have a night or two in Bangkok even, make something of it.
Some airlines, if you book direct with them, like Qatar, give you a free hotel for your layover. I go from London to BK via Doha with Qatar for about £700
You can apply for a suitable visa once married yes but you could also just apply for a retirement visa instead if you’re over 50 and live there with no need to marry at all.