Probably quite poor. FR4 boards are built with woven fibers, and you can get measurable differences depending on how tight the weave pattern is, or the angle o

Probably quite poor. FR4 boards are built with woven fibers, and you can get measurable differences depending on how tight the weave pattern is, or the angle of your transmission line relative to the weave pattern. Given that paper is not woven, I’d expect a lot more dispersion than on cheap PCBs. That said, with a good enough process able to press the paper + resin stack reliably, you could possibly get something usable.