It's been a few years now, and I realise things have changed, but I found the questions banks essential.
The first thing to accept is that you are not going to remember most of this stuff. It is a mid term memory test designed to keep pilot numbers down, and make sure that only the dedicated get through. The volume of knowledge is far too broad. The majority of it is not relevant to a type rating course. They will show you how to turn on the INS and the Air conditioning packs which is as much as you need to know.
Hopefully it has changed, but I found there were numerous examples of where two answers were correct, and you need to know which one they were looking for - the only way to do that was through question banks. Since the aim was to pass the exam, my strategy was to learn the question bank first and read the books second to consolidate my knowledge. I was training to be a pilot, not an aerodynamicist, a doctor or an electrician. My advice would be to find the way to study that gets you through, but work smart, not hard. It's all very noble wanting to have a deeper understanding of things, but you are training for an EASA licence. It's an EASA exam. If you can pass it then by definition you know enough. Trust me, it won't feel like you know anything but you have the rest of your career to study up.