So what is the your conclusion exactly? That 6Nm is not a big deal?

I believe that all very much depends on:
-the friction of the bearing blocks, in that case excellent. carrying capacity is much bigger than to have in mind in our case.
-the exact placement of the Z and gantry so the center of gravity is so that spreads the total weight of both equally on the bearing blocks.
-at what height the ball screw moves the gantry , ideally this should be the height of the cut.

if all 3 above are perfect in the design, having in mind the capacity of the bearing blocks and that we have 4x blocks, the force of the cut itself is irrelevant and even if no separation of bearing blocks all will be ok even cutting aluminum.
but as the designs are not perfect, the forces are not so straight forward, in real life the separation will help greatly especially with reducing vibrations in the machine.

So i will still separate them to form a 300mm from end to end. At the end its no more than 50-60euro worth of rails and steel for even a big machine.

As i see it, most of the designs in forums and even commercial ones, suffer from the ball screw not being where it should be, due to this and that. And it should near the level be where the cut is. That also is valid for the X / gantry / . Thats why the L gantry design and the ball screw in the middle of rails work very good.