Hmm...
share my previous plan with you.
but I am doing a resort stay so you can consider.
I believe some of the brothers & sister is doing the same way.
fly from SG - KK
stay 1 night at KK, next day morning take internal transfer to tawau.
operator will be there to pick us up at the airport and do land transfer to semporna, after which we board the boat to resort.
going back to SG will be the same in the reverse way.
it depends on the flight that you book.
I have yet to try out the flight from tawau to KL, KL - SG
Cheers~