Hey Rob,
this looks nice! A design that is very versatile!
Gantry-type CNC machines (=machines that have two Y axes) need to have a mechanism to adjust coplanarity of the machine base. If you do not benefit of the Any Surface Leveling System (that now is included with the QCW Frame, was sold separately before) in order to be able to adjust coplanarity in a very easy way, usually people have casters with adjustable height on the table’s feet to adjust coplanarity. If you have no adjustement mechanism for coplanarity, you would need to adjust it with shims, that I personally would not prefer.
The Onefinity machines already lack a mechanism for adjusting the Z assembly trimming, at least in left/right direction (front/rear Z trimming is done → this way).
You will have to ensure that your machine is rectangular (“squared”) (bar gauge) and coplanar (“not twisted”) (fishing line method).
By the way, surfacing the wasteboard does help nothing if the machine is not accurately coplanar (not twisted). All your workpieces including the wasteboard will remain being twisted if you did not adjust coplanarity accurately. Also if the machine is not both rectangular and coplanar, the Y axis movement can block or “have hiccups”.