Re-spacing the axle to better position the rear hubshell and freewheel can help a lot with any travel-limit problem in one direction or the other I've found.

Sometimes it is the particular chain and freewheel which makes the difference between shifting and non-shifting to that last position.
Stiffer old-fashioned chain may help greatly at the small end of the freewheel, while a modern Shimano chain seems to work best getting to the biggest cog reliably.

Beveling the outer face of every third tooth of the second-smallest cog can help greatly with hitting the shift to the smallest cog.

Derailers can be modified by relieving critical contact points inside of the parallelogram, using paper shim to chase down contact points needing to be ground down. This tends to be an iterative process on a derailer such as the Allvit.