Whatever you try, don't wait for route 856 unless it comes
before 169. Trust me - I'm serving my N.S. over there, and I know
the buses like the back of my hand now.
My suggestions:
1. Take 169 or 856 to Yishun, alight at the stop outside or after
the bus interchange, and transfer to either 965 or 85 (85 stops
only at the bus stop after the interchange though, and does not go
to Sengkang interchange, but it does stop at several points within
Sengkang - if you are going to a specific place in Sengkang, route
85 may be better).
2. Go to the opposite side of the road, take 169 to Woodlands, and
transfer to route 161. Be aware this takes a lot of time though, as
you are making one large round around the island. Also note that
route 161 only stops outside Sengkang Town Centre, and does not go
into the interchange.