Monthly income is only for the yearly extension. An initial Non-O you need money in the bank, unless you're embassy provides income affidavits, which most don't anymore.
When the 800k baht requirement was put in place (2018 I believe), it was to ensure those over 50 could pay for medical expenses they may incure. That's why the Non-OA that doesn't require 800k baht requires thai health insurance.