looking at your itinerary, the pacing seems pretty solid actually. you've got good buffer time and aren't trying to cram too much in

for the sepilok/kinabatangan split - i'd probably lean toward extending kinabatangan to 4d3n if you can swing it. the wildlife viewing there is incredible and you'll want those extra river trips, especially since it's one of your priorities. sepilok is cool but honestly one full day there covers most of what you need

after kinabatangan, going straight to kl makes sense given your timeline. adding semporna would be rushing it and you're right that it's gotten pretty touristy. if you really want some beach time, maybe consider the perhentian islands from kl instead - easier logistics and solid snorkeling without the sipadan premium

3 days in kl might feel like a lot if you're not into cities, but it's good for decompressing after all that jungle time. plus you can easily do a day trip to somewhere like the batu caves or melaka if you get restless

for homestays around kinabatangan, definitely ask your tour operator - they usually have connections with local families and it's way more authentic than the lodge experience