trying to keep the machine close to the cutting area

[Neale]

Here's something a bit more radical. You say you want to cut wood, but that spindle bracket and Z plate assembly looks as if it's designed to carve battleships from the solid. Why not slim down the Z plate so that, below X rail height, it is no wider than the spindle and its mounting. The mounting itself stiffens the Z plate anyway. Put just one hiwin carriage on the bottom Y rail right behind the spindle. Keep, or widen, the top of the Z assembly and use two carriages which can go over the top of the X rails for more resistance to twisting. That could buy you about 100mm extra Y travel, at a guess.

My feeling is that our size machines barely make the hiwin slides break into a sweat - they are happy to take much higher loads than we usually put on them, and one at the bottom might well be sufficient.