I put "click" flooring directly over 1.5" polyiso. it took some getting used to as it squeaked in one area by the door for the first year. but it's been down over 3 years now with no issues. do wish I'd used an additional 1/4" ply layer tho. no need to fasten. unless the 1/4" is the flooring.
My old very heavy wood stove sits on one of those big box heat shields; on top of the click floor; on top of the polyiso. there is no problem.
For another building I put 3/8" ply over top of 4" xps. feels just like a regular floor.
And on yet another building I put a layer of 2" xps with 3" polyiso on top. then used 1" white pine boards. It's still in the first year and I'm not sure if it's the boards flexing as they're not tongue and groove or the polyiso ... but there's been no issues.
your idea is solid. (sorry)
good luck.
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