If I cut a big square out of wood or acrylic  the side wall angle varies a lot. The upper horizontal cut has nearly perfect angle but the lower cut has an angle of at least 15 degree. X axis is square and bed is leveled. Mirrors are perfect. Rails are clear but I can tilt the laser heat pretty easy in both axis. The whole gantry rotates so I think the pressure of the linear wheels is not high enough. Does your gantry also tilt with some pressure?

I found nothing about adjusting the linear wheels…