I had trouble with those straps also, and I thought the buckle on them was pretty heavy so I found dogo harnesses.
They are a very clever step in harness with an extra bit of lead. I pd about $15 for each harness and bought an extra small clip for each for about .15 cents at lowes. Then I took a thin rope and tied it around the back of the seat with the clips attached and I can just un-hook the leash from the harness and clip them in the seat no problem. No harness change or fuss!
I know it sounds like a lot of extra work but it really is simple, and of course I did none of the work it was all my BF!
I get compliments all the time on the harnesses and they are so safe for our little ones!