Seems likely you can enter LOS again after a couple of weeks in Vietnam free, visa exempt for 30 days. Which, if need be, can be extended for 30 days more at your local immigration office for 1,900 baht.
In other words, you probably don't need a visa. Luck of the Irish, innit? š
No need for an agent at all; mate--- you easily qualify with 800k each. Transfer from your separate UK banks (account in her name, another account in your name) to the 2 individual Thai bank accounts you'll need to open separately in your own names, respectively)
Bangkok bank inside EmaemQuartier, Phrom Pong BTS SkyTrain station--
is easiest these days...
Get here on 60 day tourist visas; extend for 30 days in Thailand (cost 1,900 baht)
You'll have plenty of time to get everything sorted. No insurance requirement but please do get good, comprehensive insurance anyway.
Any visa exempt entry (either the current 45 day or 30 day) can be extended once at any immigration office in Thailand adding 30 days more at a cost of 1,900 thb