I have finally found a pen that works for me, and it is glorious:
It’s apparently meant for italic calligraphy, so I took a chance that would mean it will also work for the weird angle at which I hold my pens, and lo and behold, it does! I’ve tried a ton of pens before, including ballpoint pens designed for a left-handed grip, but nothing has ever quite worked right - with ballpoints I always end up having to dig into the page, with the result that my hand cramps after a while, and with fountain pens and rollerballs the ink always ends up getting stopped up in the nib, and I either have to hold the pen at a weird angle, or put up with occasionally scratching the empty nib into the page, which has to be right up there with nails on a chalkboard in the list of irritating sensations.
But this pen flows smoothly when I hold it normally (for me) and I don’t have to press down on it. Joy is absolutely the write name for it. (I’m leaving that homonymous brain fart in there because it’s excellent.)