I know Co Q10 is a good one for the heart. Adding extra taurine and feeding hearts might help too.
Also last I heard the recommendation was to continue exercise as much as the dog tolerates, but don’t push too much. Which means you may need to exercise only first thing in the morning and last thing at night so she doesn’t have trouble with the heat. And you need to be very aware of her normal gum color, so you can be immediately aware if she starts to go towards blue. You should be stopping before she is truly tired out, so you will have to learn to recognize the signs that she is starting to tire. And of course, check with your vet, because recommendations might have changed.
If you can afford it, going to the cardiac specialist one time to get a second opinion, have your questions answered, and know you are up to date on the absolute latest is really good for peace of mind.
