I have one notebook for all my subjects. This only works because I never really look back at my notes (I refer to online lecture notes, the textbook etc. for that). I only write notes down because it helps me remember. my style is similar to OP's I just sort of randomly write down whatever I think is important, I'll use lot's of arrows and pictures.

I started doing this sometime in like middle school and have stuck with it, it's perfect for me but ymmv.