There are parametric couplers out there if your printer is not handling my file well. You don’t even need to anti-backlash, it is just for those who want extra protection against zaxis tolerances. Try with out it, especially if it is already backed all the way off, just get it out of the way. Is your z nut holder in line with the coupler? Make sure it isn’t skewed. Lots of little things to check for. If your joiners didn’t print the 45 degree part well it will throw off the alignment. Could be your problem if the couplers aren’t printing well either.
Make sure your z axis is not twisted as well. Take the whole fully assembled z axis and lay it on a really flat surface like a stone counter or a piece of glass. Make sure both rods are touching nice and flat.