I am not sure if a middle click would be good for jumping, but as an alternative to the space bar it may be OK. The middle click doesn't have an in-game use anyway. So it seems to be a sensible default. But it should not replace the space bar jump by default.
But default controls are actually just a detail question. What we REALLY need here in the long run is a generalization and more customization of Minetest controls.
Two important features are missing:
1) Allowing to remap ALL actions (so that no hardcoded controls such as left click, right click, mouse wheel remain):
https://github.com/minetest/minetest/issues/45362) Allowing to manually map mouse buttons to an action:
https://github.com/minetest/minetest/issues/2361As for automatic jumping: I probably don't need it for myselves, but I clearly understand the benefits in having this feature. It will improve the accessiblity to the game for some, as you already said, the current system can even cause the hand to hurt for some. Minetest may be inaccessible to some for this simple reason.
So I am in full support of this idea. Good controls are very important for a good game.
I think nobody posted a feature request at
https://github.com/minetest/minetest/issues/ for automatic jumping far, so please go ahead and post it there (with a more complete description of what you want exactly).