We got our youngest DD a MyRide 65.
I found a good review for the Diono Radian I will post below. Sadly it doesn't make a good booster seat. I would buy it for how long you can use it rear facing and forward facing with a 5 pt harness so the cheaper option of that seat would be better.
I am curious what you mean by the straps not tight enough. I do know that with the My Ride it can be hard to loosen and tighten the harness strap. How to tell the harness is tight enough it to pinch the strap long was top to bottom. If you can pinch it then yes its too lose. I have seen some have the straps too tight you don't want your child to be uncomfortable either. Of course I have seen some so loose you can take the child out with out undoing the car seat.
In your case a 40lb rearfacing limit is just fine. I would not rear face a 6 year old. I think rear facing till 4 is fine beyond that is your choice, but you can decide better then. Also nothing wrong with a 6 year old in a booster seat. She should be just fine as long as there isn't any developmental delays and she can follow directions to stay seat properly. I see she is nearly 7 the minimum age that techs agree to start booster training is 5 in a car they are
With the My ride it can be hard to tighten it rear facing. Have you called Graco and talked to them to see what they say? If you can pinch the strap length wise up by the chest clip,