I don't know of anything that will automatically keep the focus in the
middle of the screen, which appears to be what you want. To avoid having to
use the mouse to recenter, my own solution was to assign the mouse scrolling
actions for up/down to keyboard combinations.

For me... the most logical assignments were to Ctrl+Alt+Shift+Up and Down.
So... when I press Ctrl+Alt+Shift+Down, it's the same as if I'd clicked the
bottom of the vertical scroll bar. When I press Ctrl+Alt+Shift+Up, it's the
same as if I'd clicked the top of the vertical scroll bar.

Hence, when my cursor reaches the bottom of the window, I can press
Ctrl+Shift+Alt+Down to nudge the current cursor location up to where I want
it. Similarly, if I'm at the top of the window, I can press
Ctrl+Shift+Alt+Up to nudge the cursor location back down to the middle of
the window.

I enabled this by creating two simple macros, and then assigning them to the
respective keystrokes:

For Up:

Public Sub MAIN()
WordBasic.VLine -1
End Sub

For Down:

Public Sub MAIN()
WordBasic.VLine 1
End Sub

If you know it's always going to take 10 nudges, you could replace the 1
with 10.

Hope this helps...