OP, I don’t think you’ll be disappointed if you go with the Grendel. I’ve been hunting whitetails with one since 2010, and it’s proven to be a very capable AR whitetail gun. 123gr SSTs are what I use.
I’ve got a 20” bull barrel upper and a 16” M4-barrel upper and prefer hunting with the 16”. The bull barrel is dummy accurate but probably 10# and to me, cumbersome to carry (ARs aren’t exactly smooth and svelte). I liked hunting with the grendel so much I recently put together a Howa Mini. Between my wife and I we killed 2 whitetails with it this past season. Both pass-thrus, hers dropped in its tracks, mine ran 40y spraying. Not sure what more I could ask for in a lightweight, light recoiling hunting rifle.
Around my neck of the woods I see grendel ammo regularly. Hornady mostly. $28-32 for 20 rnds seems the going rate.