The AA routefinder gives the time as 41 minutes Sheffield - DSA via Doncaster's inner ring road. This is optimistic owing to traffic congestion there; most people would stay on the M18/180 to Thorne and double back, even though this is two sides of a triangle.
The link road you mentioned to avoid this is to be built.
Carillion lands £18m link road - Highways Magazine
A rail link is extremely unlikely for reasons given earlier in the thread.