@Rainie, @whimsycwd,
An alternate solution to this problem is to use a different lead screw for the Z If I’m seeing this screw right it’s a 4 entry liead screw (meaning that there are four separate threads vs a single one) you could switch to a single entry thread which would effectively make the Z axis 4 times stronger and 4 times slower. Although because this is being speed limited by the firmware it could be made to move just as fast because it now has a better lift ratio. Maybe it could even move faster in practice with the improved ratio. You should seriously consider this change.