I don't think you're going to find Garmin to be any better. Garmin goes to the other extreme of allegedly sussing out you REM/deep/whatever sleep patterns from a HR monitor and an accelerometer. Eh, color me skeptical. IMO, Apple tells you what it is confident in telling you, and that's what time you went to bed, how many times you got up, and maybe figures out when you were awake.

That said, there are several AW apps that I've or my spouse have used that might be to your liking. I've used Autosleep (not in a while, though), and I think it might be more along what you're looking for (though you've not said specifically what you're looking for). It tries to figure out sleep patterns, and all that other stuff. More like what Garmin offers, I guess. Autosleep is pretty good at figuring out when you go to sleep without telling it. Meaning, for example, it picks up naps that Apple's built-in doesn't.

My wife has used Pillow, and it seems okay. Same as above with functionality, though I've forgotten why I didn't use Pillow instead of Autosleep.

HTH.