If you liked GW501516, give GW0742 a try. It increased rats' heart sizes in just under 24 hours. I actually have a bottle of it but never had the balls to try it.
But for both of those, and also ITPP (which was fantastic), I always had the question if we are screwing ourselves by making things like cardio easier. Without the stress of it being hard and uncomfortable, I wouldn't think the body would be very motivated to adapt and improve. ITPP was like breath in a bottle.
I actually use Semaglutide currently at 1mg per week. I never had any major side effects, and the benefits are pretty subtle at best... but where it shines for me are night cravings.