Extremely variable depending on your practice environment (academic vs community) and your role (general cardiologist vs specialist [invasive vs non-invasive]).
Fellows' life is similar to residents' life in that you perform rotations through various fields: imaging, cath, electrophysiology, CCU, consults, etc.
I think from the outside it does look "monotonous" in that every sub specialty has its bread and butter (CAD and its sequelae in cardiology, diabetes in endocrinology, etc.) Yes you're right that your patients are generally older and much of the disease is a result of lifestyle.
But in cardiology there is a lot of depth in each sub specialty; for example in imaging you can choose to learn and perform echo, nuclear, CT, and MRI. In cath, there is general diagnostics and then advanced structural work (ie valves). Electrophysiology has bread and butter pacemakers/ICDs and advanced ablations for VT/AF. Advanced heart failure is not just diuretics and med management but also working with balloon pumps/LVAD/ecmo/transplant patients too. A general cardiologist would do a little of all of the aforementioned but rely on specialists for many aspects.
There are a small portion of trainees that do a fellowship in Cardiac critical care with plans to focus their career in an ICU setting. It is niche and more likely suitable to academics since it's not a common job in community cardiology.