About the business registration, do you work for a company with an employment contract, or do you work as an independent contractor for one or multiple companies?
In the second scenario, I think they are asking for your own business registration.
Some embassies will require 3 to 6 month of bank statements with the 500K thb on the account, but people usually get asked for additional documents, they are not rejected right away for this reason.
You say "both statements combines exceed 500K THB", but remotepass is not a bank account, right? So they would not take this into account, and the other statement you provided is not above 500k, so they rejected.