So 18TB / 256GB = 70 drive writes is 1%.
For the 2T drive, we had 150TB = ~50 drive writes being 3%.
So, it seems this isn't linear after all. But it's also not constant; 150TB would put you at 8% used on your 256GB model with this scaling (instead of the ~23% if it were linear), which is still not insignificant, and would still get you to 100% within a couple years. Though there is significant rounding error with the "1%" figure.
I saw someone else mention 3% on Twitter too and asked for their drive size, so I hope that can give me another more accurate data point.