Dont do that, thinning small batches for a crack fill or such might be ok but whatever solvent you use for thinnning inevitably would ruin the properties of the resin, the more the solvent the more brittle the result, to the point of having something that performs like those roofing resin panels left in the sun for a few years, for sure there can be a compromise ratio (there always is) but i dont think it would thin enough to get the results you want.
You can find resins that are extremely thin, like some formulated for concrete foundation repairs with low pressure equipment (caulking gun type thing) they are very liquid yet they dont offer the elasticity of the resin we use, had a lot of thoughts about those when got my own house foundations fixed, since the double tube is selfmixing when squeezed out through the tube tip they would be a painless way to rig up a simple injection system but they even cost too much to be worth the try.
There are resins, specifically intended for a better wetout, that keep the viscosity to a lower level, like System 3 Silvertip laminating resin or the Interlux Epiglass or the MAS Low Viscosity. If you see that generally your own laminating technique keeps making you use lots of resin it could save some money to buy those tad more expensive but more specialized products.