I remember him saying in an interview long ago that he wrote with a fountain pen.
I changed countries and career paths about 8 years ago, and for a while, over the next 3 roles, it involved large amounts of note-taking. Tens of pages of notes per day, every day, for weeks at a time.
I switched to using fountain pens too, and found it hugely increased what you call "handwriting endurance". I can write for an hour or more with a fountain pen, without pain or difficulty or cramping.
(I am in my mid-50s now, and this was in my late 40s. That means that part of the time at school we had to use fountain pens. So it was a switch back for me.)
With ballpoint pens, be they cheap and nasty or premium high-quality ones, you can press hard. This tires the hands and causes cramps.
You can't do that with a fountain pen. It damages the nib. You have to press lightly on the paper and that makes you write better. It also encourages good cursive letter forms, which evolved for a sharp nib on paper, which snags in certain directions, also less of a problem for ballpoints.
I mostly use Pilot V-Pen disposable fountain pens. They are exceptionally reliable, the whole barrel is full of ink so they last much longer than a cartridge or reservoir pen, and they don't leak even in a backpack that's treated roughly.
I recommend trying them. It's worth the effort. It works.
Ballpoints, however fancy, are only for scribbling quick notes. A proper pen results in better writing, and writing better.