To be honest, it's not that hard to grind Cul, it's just tedious. It's entirely possible to grind with Fish -> Sand all the way to cap, it's just that the SP on that particular synth isn't very consistent so there are times you'll end up having to grind through a ton of fish for a paltry 180-220SP per, while other times you're grinding at 400-500SP per.

Now if you actually want to grind while making something useful, that's a bit harder to do.