We'd definitely organise the O visas back here in the UK then get bank accounts opened as soon as possible after arriving in Thailand as I understand it can be problematic.
The financial requirements of the LTR would be too high for us but the 800k or regular 65k would be ok. I can see that leaving 800k untouched is the simplest method but obviously we'd need money besides that to live on so if we transferred 65k each every month, even though it may be in excess of what we actually spend, it would at least then give us the option for future renewals and, if not needed for anything, we could transfer the original 800k back to our UK accounts. Having read a bit further into it though I think we'd probably just leave the 800k in the bank.