Hey All!
I have finally found an easy yet permanent fix for the headset play issue when using the separate handlebar/stem combo. Please bare with me as I'll do my best to be clear and concise. You do not need an FSA ACR setup, nor disconnect any hydraulic or shift cables.
#1 - This is NOT a design flaw on VeloBuild's behalf. The issue lies in the particular C-Clip they supply, which sits in the middle between the upper headset bearing and the black metal bearing cap.
#2 - VeloBuild's C-Clip uses two tabs in order to lock the headset cap into place. The supplied spacers also use these same little tabs/knobs in order to lock the stem in place as well. The problem with these tabs is they are just deep enough (when locked into place) to subsequently recess the metal headset cap too far into the headset bearing. This is why there's always headset play no matter how tight the compression top cap bolt is. Those little tabs are about 0.5-1.0mm tall which, when recessed into the bearing cap, make all the difference.
#3 - The solution? Using a C-Clip that DOES NOT have those little tabs. Using a totally flat C-Clip means the headset cap will still be level/flush, but it wont recess into each other by the 0.5-1.0mm mentioned above. This also enables you to use a C-shaped shim if need be, although you should not need to. Not having the recessed headset cap is basically a shim in of itself since the headset cap will sit 1mm higher.
#4 - I found all of this out while building my Yoeleo gravel bike. I had the same headset play issue. Turns out Yoeleo's headset bearings for the top and bottom are slightly two different thicknesses. The thicker (red bearing) one needs to be on top, and I had them reverse. During troubleshooting Yoeleo send me a spare flat C-Clip, but the fix was simply switching the bearings around.
The Takeaway - Confirm the headset bearings are indeed the same size (they should be) and thickness for top and bottom. Buy/Order flat C-Clip for the top headset bearing. I no longer even needed to use a spacer above the compression plug top cap. Everything is flush, and no headset play even after a 30 mile ride earlier today on terrible Chicago roads.
Photo 1 - VeloBuild's supplied C-Clip with the two tabs
Photo 2 - The replacement flat C-Clip
Photo 3 - What the flat C-Clip looks like installed
Photo 4 - My final setup with no spacer above the stem top cap