You will always have slight OAL variation and slight variation doesn't matter as long as you're not seating below recommended OAL. If you're seating deeper than what is recommended (and there is no good reason to do this), reduce the powder charge.

If you use mixed brass, the OAL variation will almost always be greater than when using uniform (same) brass. As for crimping, whether taper crimping or roll crimping, crimp just enough to hold the bullet in place both under recoil and as cartridges feed through a magazine and are chambered.

You won't get bullet setback if you do this right, but it takes some experimentation with your gun as there is no set rule on the degree of crimp. Most handloaders crimp too much and this can and often does affect accuracy adversely.

This is probably all covered in various paper handloading manuals, so you might check there if you haven't already done so. I'd avoid YouTube. Good luck-