The biggest mistake is thinking one single platform will solve everything - you need to combine sources, not keep switching between them every week. Khan Academy is solid for understanding concepts, but the ENEM tests more interpretation and context, so you should complement it with questions from past exams (which are free everywhere) and watch specific videos about the exam's trick questions, which no platform really teaches well.
People who passed usually say what matters is doing a lot of practice questions, not just watching lectures. Pick a solid theoretical base (could be Khan, could be YouTube, doesn't really matter) and then spend most of your time solving old ENEM exams, analyzing why you got stuff wrong.