In a very tall attic, let all HVAC ducts be buried under R49 insulation. Here is an example of method to protect all and to provide safety, with a maximum of flooring, safely accessible. Build always for service life of at least sixty years. Such service life adds orders of magnitude to the value of construction. Cheap construction is unaffordable. (Search my Perpetuity Math.) Build the floor 17” above drywall as dictated by economy in using 16” rips of plywood as webs in a complex box beam structure. Beam lower elements are the found floor joists whatever their dimension. Beam upper elements are typically upright 2x4s of maximum length. There is common practice to be applied in the density of screws or nails bonding webs to the 2x framing, Beam bottoms are room drywall. Beam tops are the plywood flooring, demanded for strength. Let attics be useful, with simple and sturdy lifts that add to user safety.
Such a thick floor was needed where a best attic ladder would be at 45° vs. framing.
Surrounding flooring ties to the ladder frame, with a walking surface 17" above ceiling drywall. This space is ample to move HVAC ducts to the attic floor, with safe, level, trip-free walking over a large decked area.