If you're going on to do your ATPLs, why not study to as much a level of detail from the start? You seem to be worried about a lack of a syllabus (and I have some sympathy) but the learning objectives for the ATPL exams are readily available. Have a good look at them and then you will see the breadth and depth of knowledge you need to gain.
There is much that you will disagree with having to learn (I still don't know why I had to learn which Annex for the Chicago Convention is which when it can be looked up - its not as though its something I'm likely to need in the air!) but that won't exempt you from having to do so. There is also a huge amount where it would pay to go deeper. In the end, it all depends on your approach - learn to pass the exams or be as professional as you possibly can.