This seems like they are advising you about applying for a marriage visa. Have you asked them for their list of requirements for the non o retirement visa.
It's usually very straightforward with no agent required.
I have always used Wise, but your post made me thing at a bit of a tangent.
If you used your home bank account to transfer via Swift for a large sum, would that alert them to the fact you have moved to Thailand (I ask because banks sometimes close accounts if they realise you are living abroad).
Using Wise is more 'discreet' imo, even if it costs a few Baht more.
could he return using his current extension and when it is due for renewal apply for a new non o retirement visa having deposited the 800,000 baht Brandon?
Use a card that doesn't charge a fee like Starling, Monsa and Chase, then go inside the Krungsi bank with your passport to withdraw instead of using the ATM.
You will get the overseas bank rate not the Thai bank rate and not have to pay the ATM fee.
I have seen people like you crying and complaining about the immigration officer because their visa was cancelled and they were deported. It's always someone else's fault!