I have a 1"x8"x3' solid composite board that I need to cut diagonally through the 1" dimension to compensate for an unlevel underlying surface. Is there any way to do this?
The best way to do this would be to put it on a slanted sled and run it through a planer repeatedly.
How you make the slanted sled is up to you. I would consider cutting a series of wedges from plywood and gluing them to a flat piece of plywood. You could also use shims (like for hanging a door) and nail them down to a piece of plywood.
Regardless of how you build the sled make sure that the piece you're planing is fastened securely to it (being careful not to allow your planer to hit any metal fasteners).
I'm not familiar with the PVC board material you're using but if it is uniform throughout and cuts like wood this is doable using a hand plane. Ideally you'd want to use one with a cambered iron (a slight radius on the cutting edge) to be able to remove material fairly quickly, but without that I would start with power tools and then do the final bit using the plane.
So assuming you don't have a plane fitted with a cambered iron you could use the belt sander to hog off the bulk of the material (fitted with a fresh, very coarse belt in case that's not obvious), then dust off well and switch to the plane near the end to help not overshooting the mark.
Another option that I would give serious consideration to is working on the underfloor plywood rather than the AZEK. You could work the floor in place (again with the belt sander) checking periodically by putting the AZEK down and seeing how it lays. The plywood will get chewed up a bit no matter what angle you sand it and it will look ugly as hell, but it'll be flat enough for this purpose.
I can't visualise any way you could jig this or use guides, no matter which option you go with. So either way it would be vital to stop work periodically and check fit as you go.