When my DS was about that height, the splitter plate (the metal piece that you loop the straps on) was hitting right between the back of the Radian and the cushion of my car's seat. It would get kind of stuck at a weird angle there, which would make it feel impossible to tighten. It helped to reach around to the back of the Radian and pull the slack out of the straps while tightening from the front. And it also almost always helps to tighten the straps before you buckle them if you need to do significant tightening. Pull and release a few times, then try buckling DD in to check the fit.
I am able to get our Radian straps to pass the "pinch test" every time. I would not feel secure if I could pinch any slack at DS's shoulders.
On our Radian, the harness slots on the cover do not line up exactly with the harness slots on the hard shell. It looks like I could move DS's straps up one more level because the slot on the cover is right at his shoulders. However, the straps would actually be coming from above his shoulders. Make sure to double-check that the strap height on the shell, not the cover, are at or below your DD's shoulders.
Unless DS is going back and forth between cloth and disposables or light summer clothing and jeans/fleece, I find we don't need to loosen and tighten the straps very often at all. Once I got them adjusted, I could leave them there a while. Now that he's taller and the splitter plate isn't at that difficult angle anymore, it's lots easier to adjust the straps when we need to.