The common bearing blocks are somewhere between supported-supported and fixed-supported since the bearings are not sufficiently spaced to truly count as supported:
Fixed-end, supported-end, free-end are ball-screw support conditions you need to understand in order to design the best actuator system for your equipment

You need to increase the motor voltage - 70-75V is common for Nema 23/24 motors and Nema 34 motors will work on the same, but can be operated from mains voltage with expensive drivers, which shouldn't be necessary here.

The reason the increasing the ballscrew diameter from 16mm to 25mm makes such a large difference is because the moment of inertia of the ballscrew is proportional to the radius to the power 4, so you need (25/16)^4=5.96 times more torque to accelerate it at a given rate. The gantry is driven via a huge mechanical advantage, and increasing it's mass only increases the inertia proportional to the mass (no power), so it doesn't make such a big difference.