My son has moved to a seat like this. He wasn't trying to to undo his buckle but always took his arms out, no matter how tight we did the straps. He sits nicely now, which has surprised us. Ideally we wanted to continue rear facing
Boots have the britax max safe romer on offer for £180 plus you get boots points. It's rear racing up to 36kg and 125cm. Safety wise, she really is too little to be moving to a high back booster.
They wouldn't make them if they were that unsafe, I wouldn't worry x
We’ve just ordered this car seat as we’re in the same boat, it got a decent ADAC rating and like that it’s isofix. Can always try it and if she’s going to undo the buckle can always take it back and try something else
There are rear facing car seats that go up to 36kg.. a 3 year old is definitely safer rear facing and if she’s thinking about undoing the buckle on a harness there’s no way she’ll sit sensibly with a seat belt. Ultimately it’s your decision but I’d recommend looking at an extended rear facing seat like a britax max safe pro