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 Your info is outdated and obsolete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