Malaysian Borneo specific advice for 5/6 days in mid-April

Hi everyone!

I wanted to ask you for some feedback on my upcoming trip in mid-April. I will be staying 3 weeks in total between Malaysia and Singapore. I have already arranged a somewhat good itinerary (KL, George Town, Perhenthian Islands etc.), with 5-6 days remaining to fill. I am wondering whether it makes sense (or indeed is enough time) taking a flight from KL to somewhere in Borneo (best candidate at the moment is Sandakan) and spend 5 nights and 6 days there.

I read a lot about the fact that Borneo is massive and no way I can see all of it in that little time, but maybe spend a bit of time in the Sandakan area, such as the Orangutan sanctuary, the Bear sanctuary, and a 2 nights/3 days trip across the river in Kinabatangan, doing a river safari. I am definitely interested in doing as much nature activity as possible, and seeing the local wildlife too.

My question is: has anyone got past experience in doing a similar itinerary, or should I stick to Peninsular Malaysia, add maybe an extra day in Singapore and not visit Borneo? I know that flying to Borneo can be a bit chaotic, as well as moving around there, so I am considering whether it might be worth it or not (considering flights are at least 120+EUR), as well as if it might be too complicated or tiring.

I appreciate any advice you may have!

Thanks!