If your visa hasn't been rejected, go to immig and talk to one if the IO volunteers at each office, they'll walk you through it.
'O' doesn't need health insurance.
Crim record...I think no more.
The forms are easy if you understand IO's love to fluster farang ( a power thing ), if finances ok as you say, it's just deal with frustration for an hour (or 3 or 4 ).
Went thru it over 20 years ago, renew yearly . Don't know why, but they add or subtract a form yearly, or req more Than 1 copy if you have one. If you give them 2, they shake their head and give one back.
Your UK bank wires 65k a month to Thai bank. Once a year your Thai bank gives you a letter confirming. CM immig occassionally want copies of BB pages showing 65k+ dep FFT to back up letter. ( happened twice in 20 years ).
Was a time when Consul, or Embassy prov norirized letter simplifying things.
Those days gone for many.
I don't remember ever dep 800k, but that was 22 years ago.
Everything here changes, and every IO is different.