I can't comment on Sixt or Silvercar, but a few comments:
- You say you'd need the 7 pax. Is that because you have 6-7 people or because you need the passenger space? If you have 6-7 people, you'll be tight in anything except maybe a Suburban/Yukon XL/Escalade ESV (or the Ford "equivalents"). If you have <6, then you should be ok in about anything you mention, so long as you aren't toting too many bags per person.
- While I agree that there's not much difference between the Escalade and Suburban, they're better than the QX80 in my opinion. The QX80 handles like a beast, is horrible on gas and annoying for anyone to get in/out of behind the 2nd row. The rear also seemed small in that when I've had it. The GLS is also very dated. The times I've rented them in the past year (from Avis, so your trim may be different elsewhere), they were basic. They lacked many things that Toyota has as standard on most models (lane departure, distance-based cruise, etc) and the interior felt very dated.
Sorry I can't offer much on your other specific questions, but that's the advice I can offer.