are these agents from the UK? if not why pay a Thai person to do a UK process?
Is there a guaranteed pass or no fee?
I don't understand why anyone would use an agent for this task, surely it's just a case of organising and providing the evidence the UK government request.
If they have insufficient evidence the agent can't bribe someone in the UK to approve it.
I have 2 accounts, one with the 800k deposit and a separate account for daily living expenses. It means there's no chance of me going below the visa threshold and makes it simpler to print the statement every year.
I think you would be better advised to ask in a UK visa advice group about her gaining citizenship there, but I am pretty sure your method (spouse visa several times) is the only route, I don't think there's an easy solution.
Thai ID card question depends what name is on your marriage certificate? I believe all of her documents should match (Passport/Marriage Certificate/Thai ID card), if they don't it could be tricky applying for visas. Good luck, I hope you get it sorted.