"...order pressure treated exterior plywood from his lumberyard if
they don't stock it. Spar varnish over that, ..."

The first part "pressure-treated" plywood made sense - though NOT for
Ground Contact - but the "Spar Varnish" part may prove problematic. As
I understand it PT Wood is, essentially "wet" at the point of sale.
I've been advised that coating/painting it too soon - before it has a
chance to age and dry out - can cause it to rot.

I have NEVER seen a successful application of any plywood in contact
with the ground. Though you did not indicate ground contact in the
OP, thought to mention it just in case.

Oil-based primer, at least two coats with 24 hours to dry between and
a similar number of coats of exterior enamel (latex is fine) is how I
did my replacement garage door section. But a year of sprinklers
wetting it (Florida sunshine and humidity) started the rot again.

I built a couple of barn doors with hardwood rails and stiles and a
center web made of 1/2" OSB painted the same way and they've done very
well for three years now.

If you're finished project is laid horizontally above earth or
concrete - I would suggest you create openings to provide good air-
flow beneath the structure regardless the finish.