Reading and remembering are two different skills. Most of us have practiced the first and neglected the second, which is why we can finish a chapter and not be able to say what was in it.
Use three marks and nothing else: underline what it says about God, circle every command, question mark anything unclear. Review your underlines once a week.
Marking fixes a lot of that, because the act of deciding what to mark forces you to decide what it said.
You do not need a complicated system. Complicated systems get abandoned. Here is one that survives.
Use three marks and nothing else. Underline anything that tells you something about God, what he is like, what he does, what he wants. Circle any command, anything you are actually supposed to do. Put a question mark beside anything you did not understand.
That is it. Three marks.
What this does is turn passive reading into a decision. You cannot underline a description of God without first noticing one. Within a couple of weeks you will find yourself reading with your eyes open in a way you did not before.
Then, once a week, go back through what you marked. Five minutes. Read only your underlines from the past seven days. You have just reviewed the week's teaching about the character of God in the time it takes to make coffee.
If you would rather not write in your Bible, a cheap notebook works exactly as well. The point is not the marks. The point is that you made a judgment about the text, and judgments are what stick.
Deuteronomy 6:6 says these words shall be in your heart. They get there through your hands more often than people expect.