I don't have any data for the impact of roof racks vs. rear hitch cargo options on EV range. But gas mileage on an ICE vehicle should offer some guidance. In the case of our road-tripping vehicle (shape not dissimilar to the ID.4), any roof rack cargo has a significant impact on consumption (up by a good 20% at highway speeds, more if going faster). Meanwhile, the hitch cargo solution has had barely any discernible impact on fuel economy. I'd peg it in the <5% range, which is why I'm reluctant to even attribute it to the cargo tray, as opposed to headwinds, etc.
To be sure, the viability of a hitch tray depends on your cargo. Skis may be harder to transport that way. But for luggage, etc., these are great options. When in a pinch, we just pile two of Costco's gigantic storage boxes onto the tray. They fit perfectly next to one another, and you could add more on top. Just make sure you don't cover your taillights. On the ID.4, they are high enough up to support quite some storage. Since it's not the teardrop shape of, say, the Model Y, the aerodynamics should also be impacted less.
One minor caveat is that, if memory serves, most cargo trays are meant for a 2-inch receiver, so the ID.4's 1.25-inch receiver would require some adapter.
Here's the tray we're using. Another nice feature is the ability to fold it away when not in use, presumably helping aerodynamics and reducing the vehicle footprint for parking, etc. Hope this helps.
