I would just look into calligraphy-style pens! This is definitely doable with that type of pen; calligraphy itself is an art form requiring lots of brush control to get right, so mastering this brush requires tons of practice. But you definitely can achieve the control to get both fine and thick lines out of one tool, so that’s a plus :)

Also, this sort of brush, imo, when done right, introduces a lovely organic ‘jitter’ feeling to line art.

This is a common one I’ve seen: https://a.co/d/fBBulyU