Shifting does deteriorate with usage, though it's usually the cables, I change mine every couple of years, your mileage doesn't sound enough to have caused much wear, but other factors might be relevant. Here is how I would look at it and the order of elimination.
First there's four elements, shifter>cable>gear mech>hub
You've eliminated the hub, which is a relief as that would be the complex one.
I'd take the pully out of the ex-box and pull the inner cables. You can do this without releasing them from the pulley. You should be able to spin the shifter by pulling the cables with very little effort. Then check there's no fraying where they wrap around the pulley.
If the shifter spins easily and there's no fraying, you've eliminated the cables and the shifter. Just the mechanism left, and that's been covered above so I won't repeat it.
if it's not spinning easily, you need to determine if it's the cables or the shifter.
Cables off the pulley and pulled through the outers enough to expose a few cm by the shifter. Does the shifter spin easily pulling the cables by the shifter? If yes, then it has to be the cables, you could try lubricating and cleaning, maybe pull them out and see if there's muck or corrosion. It's not a big or expensive job to replace them.
If the shifter doesn't spin easily, remove the cables and see if it does than. Shine a light down the holes and try and see the groves the cables sit in and where the cable head sits, if it isn't seated right it will make the shifter stiff. With no cables in it, the shifter should turn with the lightest of touch. if it does, then it has to be either the cables or the way they was fitted, fit a new cable and hope that cures it. If the shifter was stiff without the cable or the new cable hasn't sorted it, you'll have to dismantle the shifter, when you do so pay attention to the O rings, it might just need a clean, or you might see there's some wear, exploded diagram here
https://www.rohloff.de/en/service/handbook/speedhub/assembly/twist-shifterIt isn't as big a job as the above might seem, it's probably taken me longer to type it than it would to do it! If you're taking inners in and out, be careful not to fray the ends. If you do, or need fresh cuts to fit back into the pulley, cutting corresponding amounts off the inners and outers brings it back to the same at the cost of a slightly tighter curve at the bars.
