I'm in good shape but with a family history of heart disease. I want to quit my job soon, which means I will give up my group insurance for a few years till medicare kicks in. If I receive treatment for heart issues now because of serious chest pain, will

Uninsured. If you receive any treatment without medical insurance you are responsible for the expenses. You can take extended coverage called cobra after your group insurance expires.
Crystal ball. I know you were cut off by the character limit but i think you're looking for some way to predict the future and medicine, even stress tests or heart caths, can't predict the future. The best you can do is stay fit, normal body weight, don't smoke, keep your BP and cholesterol low, exercise, eat well, and hope for the best. We don't have a crystal ball.