Well as long as the applicant has an income letter from their embassy of at least 40k bath/month that is enogh. There is nothing saying the 40k bath/month have to be from inside Thailand or even going to a Thai bank account. My income letter only said that I have my salary from Sweden and they except it.
In sweden it only took 2 days to get my documents and when I got back after getting back from Thailand it only took 4 days to have it registered in Sweden 😁
You should check your countrys embassys website in Thailand. Usully they have written what you need to get married in Thailand. My swedish embassy have.