I, too, find it hard to work down on hands/knees these days, and if I were doing the project, I'd grout the heart first. Then, you can fill in around it after it is placed in the spot. Sounds like a fun project. Now I wish I had a hole in my front walk. Think I said the same thing when JOYY was filling holes in hers. You don't need to seal it cuz I assume you're using thinset, and I recommend you use the same thing for grout. It's concrete, like your sidewalk. Be sure and spritz the sidewalk w/water b/f setting it all in. Also, Google to see if there isn't something that w/make a good bond between old and new concrete. Seems I've read that it's necessary, but if I were doing it, I'd just spritz w/water. Then later, if it separates, put more thinset in the cracks. I experiment w/everything - totally self taught w/no fears.

I couldn't have said it any better than Slow, I agree with her that you should use thinset for grout and the only thing I don't agree with is sealing it. I seal everything.
The thing about wetting the concrete prior to filling in around this is a good idea. Good luck and post pics. I am sure it is beautiful