We own an Argos (which is similar to the Nautilus) a Maestro and a Harmony Defender for my super tall kiddo.
I will say the Argos is great as our primary seat, but I agree is too heavy and probably much more than you would need for a short trip/nanny seat and the extra funds wouldn't be necessary for that little use.
Our Maestro is used as a backup seat, so our kiddo isn't in it often, but we recently took a 5 hour trip with it and he seemed pretty comfy the whole way. Ours is not one with the harness covers, and he did get some small marks on his neck, so if that is an issue for your kiddo, you may want to steer clear (or get the one with the covers).
I absolutely love the Harmony Defender. It's a great price, was easy to install, and my kiddo seems super comfortable in it. It has the added bonus over the Maestro that it will last a lot longer as a booster. It has the no rethread harness so if you nanny doesn't reinstall, adjusting as kiddo grows will be easier. The only real downsides are: the whole base must be on the seat (not sure how deep the VW seats are), at least our version can't use latch in booster mode (without harness), and the cup holder isn't stellar.
I am actually a little jealous it is in Grandma's car and I don't get to use it often. It will probably become our backup seat when kiddo outgrows the convertible in our second vehicle instead of the Maestro.
Hope that helps!