My experience with five minute epoxy is that it's worthless compared to good quality boat-building epoxy--hardens slow, lacks strength. My experience with epoxy is glassing strip-built canoes. In that context you are discouraged from thinning because the solvents supposedly leave worm holes as they evaporate. Thus, I don't know if doing so would affect strength, but I think if you added something like glass fibers there wouldn't be a strength issue. I think there is a type of epoxy especially designed for soaking into wood to do work like restoring architectural elements that might work if you combine it with fillers.
Best,
Tom