Honestly, if "learning" is a goal, a 2mm lead holder, a regular wooden pencil, or an inexpensive dip pen are ideal. They can vary line widths with pressure in very predictable ways for low prices. The reason is that they have more "instant" snap. Whereas modern flex nibs require that you release pressure almost halfway down the stroke, a pencil or dip nib won't require it until the veeeeery end. It makes the downstrokes much more forgiving.
But the only real modern "flex" pens that come to my mind are pens with dip nib units, a-la desiderata, osprey, and flexible nib factory. A semi-flex option would be the pilot FA nib. It's soft and flexible enough to move at a good pace while still providing decent shades with good enough snapback and doesn't require that "focus" of other options.
Everything else requires a bit too much really specific feel for the pen itself. I don't think the ultraflex FPR nibs are a good way to learn, since they're not very responsive. They're more for artsy ornamental stuff, but not for everyday flare. The long slit also makes them a little prone to hard starting when writing quickly, in particular printing or writing numbers.
I don't think the pilot falcon really does much for my handwriting in stock form. I'm even a little disappointed with the flex and softness in the full spencerian customization. It's still nowhere near approaching a vintage semiflex. The angle of the nib itself doesn't really work well unless I'm going slowly. it writes well, but it's not really soft or flexy. I love the pen, but it's really inferior to the FA nib as a flex tool. Only bummer for me is the FA only comes in F. So I'm going to have mine ground to a needlepoint.
I do like the justus 95. I also wish it came in at least an EF option as well. And the feed will struggle to keep up when flex writing in cursive.
Gil, I don't follow your logic. Wet noodles have nothing to do with flex, only pressure. You don't even use vintage pens.
Edited by Honeybadgers, 04 January 2020 - 01:28.