Apparently when the conservative administration with boris was in charge the foreign office wanted the change to improve relations but the home office blocked it. No discussions atm about this but apparently the Schengen area is in discussion with the Thai government about Thais being able to holiday in that area.
It’s quite a complicated process. She will need to satisfy to the entry clearance officer that she will return. Her owning property inthai, family commitments and a stable job that will allow her time off. You will have to demonstrate your income too. I used an agent but if you’ve got time you can do it yourself.
Another idea to think about if you just want a short holiday with her in a third country is to meet in turkey. No visa requirements there.
She would need to demonstrate that she will return to Thailand. Having a job, having children there, owning property etc is good and it will cost about £100 or use. If you think this would be difficult to demonstrate why not holiday together in a third country like turkey.