In the OP, what I had meant by "550+ range" is the size of the bear, not the max distance to target. I had written that shots would be no more than 75 yards. They are generally closer than 50 yards, because outside of that distance we tend to fire a warning shot to discourage any further encroachment on the pens. With bears, this works most of the time.
I've made the decision to go with the Grendel. My AR10 is in .260 Rem, and I shoot a lot of 6.5x55, so have plenty of different 6.5 projectiles on hand. It's not a matter of switching between different projectiles for this particular rifle; my wife isn't going to ID the predator and then choose the appropriate ammo, nor does she want to think about selecting between different rifles; she wants to shoulder a dedicated rifle and go, and it will be a one-size-fits-all projectile. Likely, it will be a 120 TSX. I'm not a fan of pass-through shots, particularly because I want to be able to verify that the critter is down for the count, and I have been using Barnes for years to do this. It's usually one and done. In the .260 and the 6.5x55, I exclusively use the 127 LRX. Hasn't failed me yet.
Reloading my own ammo is an absolute requisite, which was the issue holding me back from the Grendel initially (due to lack of available brass). I'll just keep looking. I use a lot of Starline; perhaps they'll have it back in stock sooner than later.
Just for reference, the AR10 is too large for her to shoulder and get her eye behind the glass, hence modifying the existing AR15 for this purpose. It's the perfect LOP for her. The next step will be getting the barrel and bolt head on order. Remarkably, we've had relatively little predator activity so far this winter (if you can call it "winter"; I've counted at least ten days in which tree frogs have been peeping between Jan 1 and now, and normally we're averaging 25 - 40°F daytime highs; they don't generally become active until the temps are mid-60's).