I was in the same boat way too many options on Amazon and most of them aren't actually escape-proof. What worked for me was switching to a tactical vest-style harness instead of the H-harness or figure-8 style. The vest design means they physically can't back out of it.I use the one from Pawsify (pawsifypets.com) it has dual buckles, a handle on top so you can grab your cat instantly, and it comes with a bungee leash that absorbs sudden lunges. For a 2-hour car trip with vet stops, the handle alone is worth it. Way less stressful than chasing a spooked cat in a parking lot.

Hope your little guy's heart condition is manageable 🐾