Was thinking 5 nights Kuching and 5n Kota Kinabulu. Is this a reasonable first time experience? I don't want to move around too much. Prefer day tours.

That’s about the right split. Plenty of day trip options from both places. A little about our tim win both places on our blog @ https://accidentalnomads.com/category/malaysia/

If you just want to visit the two cities and take some day trips, that division of time is fine. But you may want to fly to Lahad Datu or Sandakan to see orangutans. We had a marvelous stay at the Borneo Rainforest Lodge, and were able to see orangutans in the wild (rather than in "rehab" centers).

How does Kota Kinabulu compare to Langkawi?

Entirely different sorts of places. Langkawi is an international beach resort. KK is the major city in Sabah. There are some beaches nearby, but KK is not a beach town.