I traveled in Borneo for a month, it was amazing, I did KK, then went on to climb Kinabalu, hot springs, climbed the pinnacles and visited orang-utans at Sepilok.
Kinabalu Park is fabulous, I stayed there in a dorm (super cheap)not sure if they still do that?
The flight into Mulu was a tiny 6 seater plane that flew right over the canopy of the jungle ❤️
I also had a dorm for climbing the Pinnacles at Mulu national park, that was an amazing part of the trip, you hike to camp 5, which is stunning, although accommodation is a wooden deck with foam mats 😂 The caves are stunning and the bats awesome.
This was all 15 years ago, so I can't give you any up to date info, I know it's got a lot more expensive now. When I climbed Kinabalu it was about £20 now it £200!!
Have you bought a rough guide? I planned everything from it. What do you want to do/see?
Mulu would mean an extra flight, but it is absolutely amazing and if you don't want to do a hard climb, or have children, there's loads to do.