I would get Luno life mattress. I have had them in multiple vehicles. If only on warm climate then pro is more than enough. If you are in cold climate you need the one with foam as the amount of air will make the sleeping surface very cold and rob you of heat at night.
I'm in a lot of social media overlanding/car camping spaces and I don't think one exists. Deepsleep Overland makes the type of thing you might want, but they don't currently have an option for the 6th gen according to their site (I also spoke a rep of theirs who was at Overland Expo in OC, CA who confirmed nothing was in development that he knew of).
Based on all my research and first hand knowledge, you have two options:
Remove the rear seats completely (maybe permanently?)
Removing the rear seats affords space for leveling but doesn't ensure it
As someone who has done this, I still use the Hele Box Duo platform as a base for my mattress.
If you do not remove the rear seats, you still need a platform of some sort to create a level sleeping platform and you lose several inches of height accounting for the space that the platform sits atop the physical parts. You gain storage room under the platform but that is all.
I camp a lot, and the 25 4runner folding seat design is terrible. Long story short, foam mattresses have been best , and lately I have been using a tri-fold mattress from Mattress firm but there are many brands available. I would try wayfair for options
Exped MegaMat. I use a foam kitchen mat to level out the drop. The Exped is a 4 inch combo foam/air mattress. Also get the Exped Mega pump for quick and easy inflation and most importantly, deflation. Sleep with head at rear of the car.
Similar to what PNBC4Run said, for inside we have the Lost Horizon cot size mattress - it fits exactly with the passenger second row folded down. We have a RTT, but 2 kiddos, so one person sleeps inside the vehicle. The mattress sits on top of 12-gallon bins and then we use the foam mats (from Harbor Freight) for padding/leveling.