I agree long posts are the major downside to keyboard use. Fortunately most posts are not long.
Having said that, I can imagine a possible solution. Instead of showing the red line for the entire vertical length of a post when using j/k, we could show it for the vertical area that’s visible. If you hit j again, it would then scroll to the next visible part of the post.
In other words, long posts would require hitting keys a few more times, once per visible area.