cardiologists will tell people to lose weight and exercise, but by the time a cardiologist is onboard theres usually already significant disease. you're talking about preventative medicine, which is really the domain of family physicians and general internists.
noninvasive cardiologists have some time in clinics, seeing patients in follow up or new referrals from primary care physicians. they also will typically admit patients or consult on inpatients with cardiovascular disease. they also see consults in the ED, something like new onset a-fib or heart block, stemi, etc.
it sounds like you want to do primary care. 6 years is a long time to train to prescribe dietary changes