I applaud your approach and agree with much of what you say. However, allowing students to select work which is too hard could easily be seen by senior management as not differentiating correctly. Doesnt matter that students thrive, management need to feel that there is nothing that OFSTED could complain about. I have been criticised in the past for a similar approach in my subject. I didnt change anything and merely stated that as a subject specialist with a good record of results, I will decide how I teach my subject.