you likely got an under consideration stamp, and had to return in a few weeks to get the full extension inked in. Once the under consideration stamp is given it's pretty much a given that the extension is approved.
Highlight the relevant transactions in your bank statement. Include a brief cover letter explaining your work, portfolio, and experience, goals in Thailand.
BTW, your Visa is good till August 26th next year and you won't be able to change to a Non 0 in country till after that date by exiting Thailand and re-entering
For medical you'll need documentation from the dental clinic / doctor with diagnosis, treatment plan, and appointments generally lasting 6 months or more. If you have a previous history with this clinic it might help but appointments in the past have nothing to do with future appointments.