If it's a visitor visa then the main things they're looking for is that your wife has enough money and a reason to return to Thailand after the visit.
If it's a spousal visa then, they are only really interested in your income. The income threshold is currently £18,600 but it's increasing to £29,000 on 11 April 2024. So you need to apply asap if you want the lower amount.
In my opinion it's really not worth using an agent for a UK visa, while agents in Thailand have let say "special relationships" with immigration officials they don't have any with the people that do UK visas. The other thing I would say having worked with the Home office for years is that the people that work on UK visas are looking for any excuse to decline them, even very minor mistakes can mean your visa is declined so double check everything before you apply.