3 weeks in October. Second time traveling both islands.
Want to do this trip a bit easier.
One week in the N island. Rotorua for a week? Sights around?
2 weeks in the S island. Unsure where to base x2 weeks.
I think your question may not be specific enough. It would be helpful to know where you have already been that you don’t feel a need to return to.
There is certainly a lot to do in and within driving distance of Rotarua, so it could be a possibility.
There are way too many options for the South Island for me to suggest anything without knowing more about what you are looking for. But even so, I might suggest two one-week stays, if you definitely don’t want to travel around.