On a DTV you will probably have to leave not extend because most Immigration Offices set the requirements for the one-off extension so high it is much easier to do a border bounce every 180 days.
You can't get the DTV inside Thailand so there is no conversion and not much to be made for the agent who would only upload documents for the online application in your name
You will need a TM30 from your first hotel to get the residence certificate from Immigration to open a bank account otherwise the funds will not have 2 months to season before you apply for your 12 month extension.