Dual pipes on each axis makes a huge difference. I was able to mill aluminum with 10 inches (250 mm) sticking out in Z! The overall cost is somewhat less than twice the cost of a regular MPCNC, so still less than any alternative I’ve seen, but tremendous Z stiffness.
The next biggest deflection I think is the belts stretching (depending on size), which Primo should help with, given that it uses 10mm belts and rigid anchors at both ends. Zip-ties are capable of being rigid but installation does not always achieve that.
If you already have one MPCNC I’d say make a second layer so the lower layer is Primo and the upper layer reuses your Burly MPCNC. I would bet you could achieve your goal with less redesign (although it will still require some design on your part, of the corners in particular).