So...if you're using M8 Rod....where did you get lengths from & more importantly....was it as straight as a die
Random thoughts:

Rolled threaded rod may never be straight as a die - too many stresses and no real need for it to be made dead straight.

Buy stainless steel - it's stiffer and springy so tends to stay straighter. Buy several (3)lengths in one go; they will probably be taped together which will make them less likely to be bent in transit.

Are you using servo motors or steppers? M8x1.25 will be need to be spun fast to get any linear speed - too fast for steppers without a step-up gear ?

Another option - redesign the axis to keep the thread still (taught between axis ends) then spin the nut. This will avoid any screw wobble at high speed