Hi Annyfreeze -
I understand the overwhelm on this topic. Bravo for getting after it a bit! Good for you. I'm not a doc (let alone a cardiologist) but having suffered a "Sudden Cardiac Death" episode, I'm a frequent flyer among them. I've got different issues than you (no valve replacement, few afib ablations but no structural surgery.) 17+ years ago when I dropped on the sidewalk, the guidance from the docs (post fact) was "don't even think fast thoughts!" It was pretty grim. So I mostly avoided anything more interesting than walking. Thinking in the medical community around HCM has evolved to encourage activity. Here's where I've landed (I'm pretty active but I've shifted and accommodated my age & heart issues.)
* I ride (my bike) 10-12 h week. 95% of this I do BELOW 75% of my heart rate max. The other 5% of the time (ie 30 mins) (roughly roughly roughly) I try to accumulate minutes in 90% of my Max HR. I do this through interval training & some group rides with hills that are wicked hard for me.
(I'm on some meds (including a beta blocker) so figuring out my HR max was a bit of a process. I'll add a pretty good equation at the end; run that & round down as a place to start. ) So, that's what I shoot for, but if I just dont feel like busting my hump, i'll fall back to low intensity volume. Volume Volume Volume . . . way before intensity. Bluntly, it's safer & it's more sustainable.
* I lift weights a couple times a week. Once with a trainer and once either at the Y or my home (i've got a very good set up for weightlifting at home.) There I focus on lots of reps. I NEVER do heavy heavy. Like I never do sets of 3-5 at 85% of my One Rep Max. That's a young persons game. I do 4-6 exercises (eg bicep curls is one), 3 sets of 12-15; if I can get to 20 reps, I increase the weight. I'm looking to improve strength and muscular endurance. Lifting heavy will just wear you out, drive your blood pressure up (probably bad given heart stuff), is less sustainable, and wont get you the results you're hoping for. Strength and endurance are what you're hoping for. You dont need or want to lift heavy for that. Working with a trainer once/week or so to ensure you're executing good form will be safer than trying to sort all this stuff out on your own. Work out with a trainer once & have them put together a workout for you to do solo 1x/week. I love the Y. Lots of normal people and their trainers aren't wicked expensive.
* I do a sorta strength class at the Y 2x week. Lots and lots of reps. Almost a cardio HIIT class although my HR never gets above 60% of my max HR. Lots of body weight moves (eg planks, pushups, etc). I look at that as good "functional strength" . . . lots of movement, lightly loaded if at all, . . .

So, my suggestions are;
1) Join the Y
2) Get a trainer 1x / week
3) Sign up for a class 2x/wk where you can meet some folks. Make sure you're ok doing it half-assed if that's where you're at on any particular day. Modify the movements, don't work as hard as you could, . . . but go. "Anything worth doing is worth doing half assed!"
4) Walk 4-5 days a week for 30-60 minutes. Dont worry about looking like a goofball with handweights or whatever. Just get out and walk. I'd walk for 60 days then MAYBE start thinking about how fast you're doing it.
5) My doc used to tell me "you can't out run the fork." I really didn't like that. And yet, somehow it's true. I exercise 12-16 hours/week (more in the summer with longer rides) and I still have to watch my food intake. Sucks and yet, it's still true.

The half assed thing is legit. Sometimes the biggest win is just walking through the door for the class. In my case it's throwing my leg over the bike knowing I'm gonna put an embarrasingly low amount of effort into it. And, often, not always, I feel like working a little harder. Sometimes I dont. But I always have a ceiling on effort related to my HR (below 75% max HR in most cases)

Keep in mind that three 15 minute walks on three days is better than one, hour-long walk on one day (and HoHo's and ding dongs the other days!) I'm a believer in 1) do the work; go through the motions. 2) no pain, no gain is ridiculous even for teenagers. There's no science that suggests thats smart. NONE.

Focus on volume. Stay away from the HIIT stuff. It'll just wear you out (best case) or you'll hurt yourself because there's plenty of 25 year old class instructors who aren't focusing on making sure they're people are executing with decent form. Get the volume (low intensity) in for at least 90 days. After that, maybe you can do a HIIT class because you think it'll be fun or you want the challenge. The P90X stuff that Dale mentions below is mostly high repetition/functional/strength and endurance. You might like that. Lots of people do. I'd find a class where you can start to accumulate a "tribe" of folks you like and enjoy that are all in their sweating like dogs just like you!

Here's some stuff on the "Polarized training" (bulk of work at low intensity.) Stephen Seiler (American endurance research scientist that’s been working in Norway for ~20 years; https://www.fasttalklabs.com/training/polarize-your-training-stress/)

Here's three ways to estimate max HR. I use #2 and ROUND DOWN
[ 220 – Age ] – most common and widely used maximum heart rate formula
[ 207 – 0.7 x Age ] – more precise formula, adjusted for people over the age of 40
[ 211 – 0.64 x Age ] – slightly more precise formula, adjusted for generally active people

An apple watch (or a garmin watch), fitbit, etc will give you insight into what your heart rate is while exercising. The most precise way to do this is by connecting that watch (or bike computer in my case) to a Heartrate Strap. To my knowledge, the Polar H10 is the most accurate strap on the consumer market right now. Garmin makes straps that are also good (I've got each.) I'm sorta neurotic about paying attention to my HR. That might just be a me thing . . . and it took me about 6 years of regular exercise paying attention to it before I became convinced that I wasn't gonna have another cardiac arrest. 😉

Hope that helps
Best,
John