What I really want for spacebar scrolling is this in working: https://greasyfork.org/en/scripts/20937-red-read-line

An indicator on where the bottom of the screen scrolled to, so I don't need slow animations or incremental scrolls to find where I left off reading.

(The linked script is not working on all pages and sometimes blocking klicks, I just threw it together to try if the Idea works. I think the principle works really really well.)