Where are you? Bangkok, Chiang Mai, and Phuket you can apply for extensions up to 45 days before your current stay expires. Most other offices is 30 days.
You cannot open a bank account from outside Thailand. Non-O visa obtained from a Thai Embassy in the US can be in a US bank account. Remember, a Non-O is a 90 day visa. Once you enter Thailand you'll need to apply for a 1 year extension, which requires the 800k baht to be in a Thai bank for 60 days before applying.
45* days before your stay expires you can apply for another extension. If this is your first covid extension, the 60 days will start from the day you apply, so I'd wait until the end of January.
Depends which immigration office, Bangkok, chiang Mai, and Phuket you can apply 45 days in advance, others it's 30. But for the covid extension your first one will start the day you apply (at most places it seems).