Unless you have really good dust collection, it will also make your workshop
> look like a snowstorm took place in there.
Test this out first by sucking up a dozen gophers and a couple of Grateful Dead fans through your dust extraction. If it copes with this, it'll cope with the swarf from HDPE. Certainly when I bandsaw the stuff I don't use the dust extraction - there's a wire guard in there and HDPE swarf catches straight onto it (or any protrusion) and makes the dust bunny from hell.
I'd be a bit worried about this thing sagging if left loaded for a long period. HDPE will creep over time. Rather than thinning it down, I'd machine it out on a router. If your insert is anything like mine, then 3/4" thickness is nothing to it, except for a narrow strip over the arbor and four little mounting pads in the corners.