Effi,

thought of shimming the rafters down (making them thicker) but didn't
like the idea of heavy hardie panel ceiling attached to 2x4's attached
to 2x4's, so thought about running 2x6's alongside the existing 2x4's,

The biggest problem I see with that arrangement is the overall thickness of
the doubled up rafters. The wood framing is the weak link in the
insulation. In simple terms, the space taken up by the rafter is
uninsulated. If you double up the rafters, you'll have twice as much
uninsulated space (twice as much heat loss).

If you used construction adhesive and long screws every 12-16" or so to
attach the shim boards, I doubt you'll have any problems. I don't know what
a Hardie-panel weighs, but even if it weighs 100 pounds, that's only a bit
over 3 pounds for each square foot (4x8 sheet). With 24" OC rafter spacing,
you may end up with 7-8 pounds on each linear foot of the rafter. Not a big
load.

Alternatively, or in combination, you could glue/nail plywood on both sides
of the rafter and shim board. In effect, you would be building box
beams. This would actually allow you to increase the rafter depth as much
as you wanted. In fact, you could provide an air gap between the rafter and
shim board, which would reduce thermal bridging. You could even insulate
that gap if you want to get really efficient.

wood blocks 1" x 1" square x 1.25" thick glued to underside of plywood
roof decking staggered every 12 or 18"

Sounds like a lot of work with little gain. Increase the insulation space
and use standard plastic insulation baffles if you need them.

radiant barrier (perforated brown paper with aluminum foil facing on
one side, stapled to thel 1".25 thick blocks of wood

If you use a radiant barrier, it should be on the heated side of the
insulation, not against the roof. You want to reflect the heat back into
the room, before it goes into the insulation.

(unless you live in a warm climate and are more concerned about keeping the
heat OUT of the house).

may cut some leftover 24" wide 8 1/4" thick encapsulated fiberglass
in half using electric meat carving type knife

Sounds real itchy... Easier to just lay the mat over a board, compress it
with another board, and a quick swipe with a sharp knife. You'll end up
with a cleaner cut and less fiberglass dust floating in air. Be sure to
wear gloves and dust masks. Insulating is miserable work...

Anthony