Honestly, this why I don't ever take my shyt down to skin no more. No matter what I try I get bumps.
I got my regiment tho, might not work for no one else.. but anyways, I use clippers only. The day of the cut I spray some alcohol on. Make sure the area is clean and dry, not oily. After the cut, alcohol again, warm rag face wash, and coconut oil.
This greatly reduced the bumps. Still tho, it ain't perfect. I only cut my hair down once a week max, and line it up a few times between to stay fresh.
I don't actually like having a beard, shyt has its own problems, but keeping it at zero is far worse. Just keep your shyt low and lined up.