I have a RAV4 with similarly hard seats.
And I really hate installing seats in that suv. The tricks I've found that give me a good install within 5 minutes or so are:
1. Recline the rear seats. I don't know if the 4Runner seats recline at all, but the RAV seats do. I recline them, get the seat in as tight as I can (it still moves), then put them back straight upright. The seat is in rock-solid.
2. Twist the female end (buckle) of the seat belt. I've found that the Toyota buckle stalks seem to be fairly long and they put the buckle right at the point in the belt path of the car seat where it interferes with getting a tight fit. You can twist up to 6 times, but likely will only be able to do 3 at most.
3. I stand on the seat and brace my back against the ceiling, then push down with my legs as I tighten the seat belt. This is definitely easier with 2 people. Rocking side-to-side or bouncing can help as you tighten the belt.
I'm also going to guess that the base of the AO is interfering with the install since it's so big. Try taking the base off and that will probably help a great deal.
Also, use a pool noodle or tightly rolled towel to help with the angle. Put it in the 4Runner seat bight--the crack where the back cushion meets the seat cushion. You may need to use more than 1 to get a good angle. Since your daughter is older, she doesn't have to be at the 45 degree angle that's recommended for newborns. Just pick a good recline that's comfortable for her and allows her to sleep without her head falling forward.