Earlier quoted context omitted.
You are over estimating car seat weights. A large, one piece, high end seat for kids up to 70 pounds like a Britax Marathon only weighs 19.5 pounds. No pixie dust needed, just plastic and styrofoam. https://us.britax.com/car-seats/marathon-70/ (see specifications) Those detachable seats you see mom's carrying in the supermarket weigh about 10 pounds, and the base will be about the same.
The one you link to is a) not ISOFIX, thus not high end, and b) discontinued from the manufacturer. We have this one, which is what I'd consider the gold standard. It's ISOFIX, and only rearward-facing since kids under 4 should not be facing front, and it's won basically every test there is: http://www.besafe.com/product-int/izi-kid-x2-i-size/ It weighs 40.6 lbs. Quite a bit of structural steel in there, including a…
https://us.britax.com/car-seats/marathon/https://us.britax.c...
So maybe in Europe you are going to have a harder time with self driving cars because you need heavier car seats, but you'll probably find a workable compromise. Yu can just build your base into the center seat or something.