I use dtac, get a cheap tourist one at the airport (they will register it for you)then download the app and select what package you want. For international calls they charge 7bht a minute to landlines
Salary doesn’t matter, if you’ve got the 500k equivalent in your uk bank account can one of your employers write you a short letter stating that some of your work can be done remotely? Thats what I did through London embassy.
It is burning season soon up north which drifts smog across most regions, please consider this as well if you have breathing issues. Particularly bad in chiang mai areas