We decided to get Amy a proper bicycle now.
She wanted a blue one, because blue is her favourite colour. Fair enough. But she also wanted one with dolphins on it! This we couldn't promise.
But. Stella found a blue bicycle with dolphins on it, in Amy's size! So we got her that.
And after only a few days, she could already ride without assistance. (I'm certain that her experience with the balance bicycle helped.)
She needs help getting started (getting on and getting moving), but once she's under way, we can let go. And she still needs to learn to brake properly (using the hand brake or ... what do you call it in English? backpedalling? where pedalling backwards brakes instead. Rücktrittbremse.); right now, she still brakes using her feet, like she had to do with her balance bicycle (or her Bobby car), which had no brakes.
And for those of you who would like an image, here you go: