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 full integral roll bar and a brace against being rear-ended, both of which your linked Britax seat lacks.
Also bloody expensive, but if there's one thing I won't skimp on, it's the safety of my children.
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.
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.