its really simple and on the evisa website. You simply leave Thailand when your date valid till stamp in your assport shows. So your answer is leave the country and re-enter on 180 days or before
never but visited many dental surgeries here and much better than Australia. Unless your dental problems are severe I suggest you use google maps to find the one close to you. If bad problems then I suggest the Bangkok dental hospital or the dental surgery attached to @King Chulalongkorn Memorial Hospital.