with the OA you’ll need police background check,medical certificate and you’ll need basically not very good Thai insurance and yes you can use the 800k in your US bank
your funds must show as an international transfer taxed or not you can start this immediately and use it for your second year extension the 800k has to stay in your account for 5 months then it can drop to 400K then top it up to 800K 2 months before your second extension use your monthly for that once you get the stamp do as you please with the 800K
get your non-o visas in the US then you can open separate bank accounts here get your 800k baht in each one seasoned for 2 months and get the one year extension