I tested several rear-facing seats in a prior car. These measurements were all with the rear seat configuration in a 4Runner but should be generally true for the Y. I'm not surprised you had issues with the Britax--they're ridiculously inefficient on space and weight.
Nuna Pipa cradle in detachable base: This was my baseline. It could just fit behind the driver's seat in the 4Runner with the seat in a comfortable position for a 5'9" driver.
Graco Extend-2-fit: Winner of the test by a good margin. Gained 2-3" legroom over baseline, even in the middle setting on recline. Base of the seat is nicely sized for more compact seats. Reasonably well-made. Has an extendable leg platform for taller kids in rear-facing mode, but will cost you legroom for the front seat. This seat can go in rear-facing configuration behind the passenger seat and would be comfortable for passengers under around 6' or so. Extra bonus is that the adjustable headrest is angled such that it won't hit the passenger seat when it is extended, so it could go all the way up if needed without effecting fitment (though a kid that tall would have their legs crammed).
Chicco Nextfit: Within +/- 0.25" of the Graco depending on how the front seat headrest is designed. Feels sturdy but has extremely high bolsters that will make loading a kid through smaller doors difficult. This seat would be a headache on a car with a low door line.
Safety 1st Guide 65: Lost about 1-0.75" of legroom from baseline. This is a smaller seat, but the base design makes it inefficient overall.
Safety 1st Grow and Go: Similar to the Guide 65. Has two recline modes to save a little bit of space, but the higher of the recline settings seems too steep for younger children.
Britax Boulevard: LOL, I can't believe some car seat blog had this listed as the most compact car-friendly seat. Lost over 2" from baseline and the seat is just huge and heavy. Questionable build quality (recline handle was already sticking after just a couple uses) and the most expensive seat I tested by $70.
