Brush pens supply far more variation in line width than the usual fineliner pen. Their nibs have a wide foundation that then tapers to your wonderful issue. Normally the nib is made from a conical bit of felt, but you can find pens that have a real brush idea https://ink-pen-drawing-technique09876.thechapblog.com/34200800/about-how-to-draw-with-ink-pens