I also like to just kind of scribble and doodle for fun from imagination but I'm not very good at it. Everything I draw that way tends to look very cartoonish and overly simplified in terms of perspective and form and I'm very messy with the pen drawing this way, trying to sort of search for the right strokes while scribbling all over the place.
Similar kind of doodle from memory/imagination with graphite. I find graphite and charcoal very tedious with how they smudge. I generally just prefer to scribble with the pen:
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