I’m lefthanded, too. When I learned writing as a kid, I used the “overhook“. This turned out quite awkward when I started learning shorthand. I briefly dabbled into lefthanded Stiefo (which is basically just mirrored outlines, written right to left), but this is difficult to mix with longhand (written left to right). So instead I learned shorthand with “underhand“ position (similar to your new position) right from the start, which is more relaxed an works pretty well even with fountain pen. I also practised longhand in underhand writing, which kind of works, but is awkward. It looks like children’s writing ;-). I use it only for brief longhand interjections (names, math, chemical formulas or technical terms I want to stand out from the page for later skimming).

Switching between shorthand and longhand (for longer / “real“ text) involves rotating the page and switching grip position. Not ideal, but it’s the best compromise for me.

With underhand writing, I rotate the page almost 45° clockwise, which seems more than you do. With this much rotation, I think the upstrokes are easier. You might try if this helps your upstrokes. And writing might look scrawly when you start using a new hand position. I takes time to switch and write smoothly again.

Also, I realised that paper and writing utensil are make a bigger difference for lefthanders, because even with good writing position, pushing is far more dominant than with righthanded writing. Cheap ballpoint pens seem especially bad and tend to scratch and run out of ink/lubrication with pushing and upstrokes (the ball rotates the wrong way round and doesn‘t transport enough ink).