A liquid ink rollerball pen, for example the classic "uni-ball Vision Elite" is even closer to the effortless writing experience of a fountain pen (i.e. with a minimum pressure on the paper), than any gel pen (but gel pens are still much better than traditional ball-point pens, which require an excessive pressure on the paper; fountain pens and liquid-ink rollerball pens need to just touch the paper, almost no pressure needed).

Versus a fountain pen, a liquid-ink rollerball pen has the advantage of being insensitive to orientation (which may be difficult to control if you tire easily) and it is also much more unlikely to dry up when unused.

I do not write so much by hand as to become tired, so for myself I prefer a Japanese fountain pen with a nib that allows variable-width lines, for less boring letter forms, but if the criterion for choosing is the maximum possible comfort then I recommend a liquid-ink rollerball pen, and I have experimented with all kinds of writing implements known to mankind.

Except for using the most comfortable writing tool, there is nothing else to do besides changing the style in which you write, to simplify the hand movements.

Perhaps you should alternate between 2 different styles of handwriting, because they might stress different hand muscles.

I have no idea what style of handwriting you use, but I am not convinced that the kinds of "cursive" handwriting that were taught in many schools are really the least tiring, despite their emphasis on minimizing the number of times when the pen is raised from the paper. Many years ago I have abandoned the kind of writing that I was taught as a child and I have switched to one of the variants of the so-called "Italic" handwriting, which are now promoted in many places and which are a revival of the Aldine letters from the Italy of the 16th century, which have also been the base from which the so-called "Italic" style of the printed typefaces (fonts) had been developed. Such a handwriting has improved readability, but how fast and comfortable it is felt, can vary a lot between people. It works for me, but that does not mean that it would also be the best for others. There are books and on-line tutorials about this.

Perhaps you should just write slower, to avoid fatigue.