One of Nasus's problems is that he doesn't have a lot of skill expression; all he can really do is decide if it's time to all-in someone. His abilities are very binary, there's an obviously ideal way to use them.
His high stats and high predictability are why he's so strong at low tiers and so mediocre at higher tiers. While increased numbers on his ultimate will definitely make him stronger, it won't solve his fundamental problem.
In my opinion what Nasus really needs is some kind of rework, rather than number tuning. I understand that's a limited resource, but a fix could be as simple as putting some kind of new effect on his Spirit Fire. His Spirit Fire is probably his ability with the most untapped potential, as it's a skillshot with an area effect. There's a lot of gameplay that could introduce, but right now it mostly boils down to throw it on whoever you want to kill and try to keep them in it with Wither. An example change that comes to mind is giving Nasus a movement speed bonus or slow resistance in his Spirit Fire, so there's a decision of whether he wants to use Spirit Fire to help him engage, or if he wants to save it for when he's on top of them for the armor shred.
Another way to introduce skill into Nasus's kit would be to nerf his ultimate's flat resistance bonus and put armor/mr scaling in its place. This might sound stupid, but less experienced players don't necessarily know what damage type their enemies deal, or how resistance items work. Introducing scaling into Nasus's ultimate would put a very basic game knowledge check into his kit, rewarding players who are able to be flexible with their builds and recognize what items would help them the most against the enemy team. (it _might_ also help guide brand new players into tank builds)