is all the netorase optional?
Strictly speaking, yes, however, if you avoid all the netorase content, you're basically avoiding 95% of the H content of this game. For one.
For two, you're going to have a hellishly hard time going through the dungeon, because power ups for SP will be extremely slow, and on top of that, you won't be able to unlock the better equipment at the weapons and armor shop... and dungeon drops are... EXTREMELY unreliable...
You also won't be able to unlock the Alchemy enhancement and crafting stuff with Sana... I think. If my memory isn't failing me, you need to do that one quest where Kai ends up fucking Saria and MC fucks Sana to have access to all the stuff.
But I'm not sure about that one. Been quite a while since I played the game...