There's a good post in the Internal Medicine forum about what "top" cardiology (adults obvi) fellowships are looking for. Might be useful to read, keeping in mind that there are some significant differences between the peds and adult worlds.
The answer to all your questions is "it depends"...and unfortunately, there is an element of timing that you have no control over that may make or break your ability to find your best fit.
In general, cardiology is not considered one of the lifestyle fields (especially in fellowship) but to be honest with you, "lifestyle" means a lot of different things to a lot of different people. For some of us (the PICU/NICU/PEM folks come to mind), the thought of being at home attached to a pager or at the beck and call of nervous moms all night when we're trying to spend time with our families is untenable. We much prefer to leave work at work and when we're out of the hospital others are taking care of our patients, even if that means that we give up our nights, weekends, and holidays perhaps more often. For other people, not spending the night at the hospital is of the utmost priority and even if there is the risk of a phone call (or even lots of them), that's better than the alternative. So where your ideal "lifestyle" fits, may be perfectly amenable to any number of sub fields within cardiology or none at all.
Additionally, pediatric cardiology tends to attract a lot of superstars, and the need to be academically productive is probably higher than in any other pediatric subspecialty across the board. Writing papers takes time, mentoring fellows takes time, and these activities may fall outside of standard clinical hours. When my manuscript got accepted for publication 6 months after completing fellowship, I had a 48 hour turnaround to submit the final proof before it went up online...which meant that on Saturday afternoon I was on the phone with my mentors getting them to review it for the 87th time (and we still found things we wanted to change). So while you may have less service or clinic time in an academic position, there are other things that can demand your attention challenging that idea of lifestyle.
No one in pediatric fellowships ever talks about private practice, particularly as you go to bigger and bigger name institutions. If they do it's in hushed tones with lots of innuendo and in some cases derision. If you walk into a fellowship interview stating that's your primary goal, it's probably the fastest way to find yourself at the bottom of their rank list. As someone who joined a private practice PICU group, I can tell that there's a little bit of disappointment from my division head and fellowship director that I didn't add my name to the list of faculty members at premiere institutions. But I'm much happier and better compensated.
That balance between PP and academia impacts lifestyle in other ways. A pediatric cardiologist in PP can probably do well enough in any town with more than 150k people, particularly if there is a sizable, acute enough, NICU (. But that probably is a solo practice or at the most a 2 provider sort of set up. That may create a very different sort of lifestyle and different sorts of responsibilities. Bigger cities of course have more opportunities, but also tend to have academic centers to sweep up all the volume, so you have to find a unique niche and symbiotic relationship to function and recruit patients. Further some of the larger academic centers (eg CHOP and Northwestern/Lurie Children's) are fanning out into the distant suburbs and exurbs where general cardiologists may have been able to make a foothold for the routine stuff, creating relationships for any and all types of pediatric care. Depending on the needs of the community hospitals and the academic unit, you may find a perfect fit or feel completely overwhelmed by the bureaucracy.
Your assessment of EP and Interventional are pretty accurate but it's hard to know what things may look like in 6 years. People have been selling Interventional's scarcity for a while a now, that it's only a matter of time before the pendulum swings back the other way.
MonkeyRalph's description of the job markets is pretty spot from my vantage point - although I haven't meant many pediatric cardiologists that love pulmonary hypertension. 4th year CICU/dual PICU and cardiology folks are in demand EVERYWHERE right now, but again, in 6 years it's very possible that every place that can add a Cardiac Intensivist will have done so. 4th year fellowships are nice, but are very much a dual edged sword (see the adult cardiology thread I mentioned above), as having a niche makes you both more and less marketable. University ABC needs an EP person, with particular expertise in complex arrhythmia procedures for some sort of multispecialty clinic they want to open...might be your exact interest but they post the job in February of your 3rd year of cardiology fellowship instead of September of your 4th year EP year and so you're out of the running by the time you have demonstrated the credentials they need. Now everyone you talk to wants EP people focusing on genetics and channelopathies and you're left moving across the country for a position that kind of fits but not really. And depending on your sensibilities, you may have trained yourself out of a more general cardiology position either because of your interest or not wanting to feel like you've wasted a year of your life by not fully using that training.
TL; DR - pediatric cardiology is much less likely to work out for someone "not interested in a field where I'll be working long hours or with lots of call" than other fields in Pediatrics
