smooth(Heading,clamp01(Time<1)*10000)+smooth(Time,cos(RollAngle)*YawRate+sin(RollAngle)*PitchRate)

This is as close as I've gotten it, it works for the most part but until some point, either in time or in movement it starts to drift by a few degrees.

Looking at it now

smooth(smooth(Heading,clamp01(Time<1)*10000),cos(RollAngle)*YawRate+sin(RollAngle)*PitchRate)

simplifies it a little bit, but still has drift