I misread this post
@J**
:O Now I see we're talking about you getting the extension NEXT year (in 2022) not an extension this year :P
So as long as you HIT the "money stays in the account for 3 months AFTER your current extension was granted" then you could transfer it in "chunks" over to the new bank where the balance never went below 400K on the main account until you had at least 400K baht in the other account you should be fine.
When is your 2021 extension due?