I think you are right in understanding doing cardiology, MICU, and interventional isn't feasible. When you do cardiology you are entering a whole new field in some respects which has multiple further subspecializations. Cardiology has become much more complex over the last decade that general cardiologists sometimes don't even do angios anymore (at least at my shop, a huge cardiology academic center). It's just too busy for them on wards and clinic and teaching, etc.
So if you decide cardiology you are committing yourself to the heart/vascular disease and related conditions. As a cardiologist you will still see a lot of unrelated pathology however in both the clinic, hospital wards (on your primary service), and on consults for stuff not even related to the heart. You won't be the go to person for most of it but you'll have to deal with it and r/o, for example, lung pathology as a cause of symptoms. One example, cardiology called on a patient for syncope by ED... subsequently they discovered a brain tumor.
If you decide interventional your life is in the cath lab, clinic, consults, and possible some ward pts depending on where you are. It's going to be mostly procedures day in day out.
You already know about MICU. You don't have to do pulm/cc to get certified as cc though usually in MICU I think it's probably more ideal to have the pulm/cc background if you want to work with the complicated cases. Lots of big hospitals have separate ICUs for all sorts of conditions and the "medical" patients, aka super sick ones, are sent to the MICU which are usually staffed by pulm/cc guys/girls.
But that being said cardiology has CCU and there are plenty of cardiologists doing mostly that. You'll still get a good amount of "medical" patients because there will be overflow to your unit when the micu is full.
To me it sounds like you should do cardiology because that is ultimately where your passion is. Rest assured you will still see a lot of stuff. But you will lose a lot of your generalist training after years and years of just doing cardiology. My shop has a cardiology rotation where we are primary. ED was usually pretty good at getting us appropriate pts but when it wasn't related to the heart we were calling lots of consults, and that's because the attendings and fellows were not that good at unrelated conditions (which is fine because they are not trained to be good at unrelated stuff). A patient I saw in clinic yesterday was seeing about 6 specialists for problems which could all easily be handled by a pcp... he just started seeing endocrine for DM with HbA1c of 6.8 and started on januvia... Cardiology punted that management. But that is ultimately the way it is with most subspecialty people. You cannot be good at everything.