She needs a better car seat than this in general. A good budget option would be one of the Joie seats. In regards to getting her arms out, the only tested thing you can use is the besafe clip, anything else has not been accurately tested therefore should not be added to car seats
A booster seat is definitely not the answer as if she can’t sit sensibly in a car seat she will most likely try to take her belt off in a booster too. If she can take her arms out the straps (which is something you should worry about, they are what will save her in a crash) she most likely isn’t fitted correctly in the car seat. You can try putting Velcro on the clip as that will make it uncomfortable for her to unclip