Step Two Point Nine Five, V 2.0
Did a little better with the second. If I were to pursue this again, I'd probably cut the plugs shorter first. It doesn't seem to be related to the adhesive, as I "dry turned" another one and it pretty much stopped in the same spot. My plan was to then drill out the plug, and run an M6 bolt through it, directly to the rack.
Upon closer inspection, the side threaded bits of the rack were actually slightly recessed, so this system (or using a dowel or plastic dowel) would need to be fitted or shimmed.
So, the plug was pulled on this idea.... leading to...