. .and if the relationship gets sour, all these leases, lawyers, paper documents are not helpful in anything, since you don't want to stay living in "her" village, in the midst of people who will eventually start to dislike you. And you don't want to become the victim of an accident that happens because you are standing in somebody's way. The saying is: "invest in Thailand only what you can walk away from anytime, without a blink of your eye"
are you aware that a lot around these proceedings has GRAVELY changed since February 2025? Please ask yourself: is your knowledge up to date? The last price quote for what you suggested, came from VCT, it was 55,000 Thai Baht. And a month ago they posted, that they cannot offer this service any more. Your info is outdated and obsolete
if he uses monthly income affidavit method, it is only 40,000 THB (1065.- Euro) per month for the Non-Imm-O family visa and the Extension. His monthly 1700.- Euro pension is far above the required amount
you will be fine. And even if not: transfer your money back to your UK account from the Thai bank. Then use WISE to transfer it back to your Thai account, and choose as reason in the list "funds for longtime stay in Thailand". This transfer will be coded as having come from abroad