I will shy away from NicNat because my assessment of his PIT based scores is low. Let me explain.
I assume you do not want to trade each time NicNat is "rested." A defender or a forward has two bench players to cover an absence. A midfielder has three. Typically we look to have the better rookies as bench cover as they await harvesting. Hence they provide good cover if an elite or premium has a short term problem. And a reasonable PIT.
Now, consider who is your ruck reserve. For me last year it was "supersub" from the West Coast. He never played but he was an excellent vice captain. Sure, he was useful to switch with a F/R if necessary. But, I still have bad memories of sticking with Westhoff in the forward line as ruck cover. Never again. The points I had to forego just kept rising (except for round 1). My ruck PIT number was either zero, or negative. Man that hurt. This year I'll go with a low priced F/R whose club plays a lot on Sundays. Comben from North probably.
So set and forget for me. If Grundy or Gawn were to get injured, I can trade down to the next best performing ruck. And, you'll almost certainly have the capacity to make that trade if it is necessary.