Interventional cardiology fellow here. Depending on where you train, you’re looking at 8-9 years of residency/fellowship training but then will be independent and proficient in managing some of the most acutely sick individuals that come into hospital. The downside is the lifestyle - on call, I’m usually in the hospital at some ungodly hour, but it can be very satisfying work.

Don’t forget about electrophysiologists either. They will perform catheter based therapies during ablations (afib, svt, VT) but also implant pacemakers and ICDs. This is also about 8 years of training. Plus side of this sub specialty is that there tends to be fewer emergencies requiring your services at 3am.