You do not need to season the funds to get an O-visa. Seasoning is only required for extensions, and note that the required period is two months - not 60 days.
For O-visa (retirement) the 800,000 baht must have been brought on from overseas.
You need two weeks left on your current stay (three weeks if applying in Chiang Mai and perhaps some other provinces).
Every country that has an embassy here issues those letters, except for Australia, USA, and UK. Canada later stopped issuing the letters too.
Denmark also stopped issuing the letters, but if you are on a Danish social pension, the Danish embassy will stamp it "Document confirmed genuine" after they check with the Danish social pension office.
I think for Canada and Denmark it'd be best to check the current status.
You should be able to get non-O (marriage or volunteer) in your home country. But non-O (retirement) depends on which country. For example, USA: "yes"; Australia: I think "no".
I think these are all 90-day visas. You do not have to do any border run when extending. Once you have arrived on such a visa, it is just the same as if you had obtained it inside Thailand. i.e. you can easily extend it every 12 months.
There is(was) another type of O-visa (marriage) that was obtainable in Savannaket and HCMC that was a one-year visa with 90-day stay each time. So that visa pretty-much required a border run every 90 days.