The full length of the DRL from Spadina to Steeles needs to be built as one project, but open in phases.
Phase 1 to Danforth with Eglinton as phase 2 follow by Sheppard as phase 3 and Steeles as phase 4. You could break it down more with phase 4 being Finch and Steeles as 5. You could still have more phases and that needs to hapen from day one as to what they are.
Stopping DRL at Danforth offer no improvement to the Yonge Line. Eglinton will add some improvement, but taken up by new development around the Yonge station.
Again, if you extend the DRL to Hwy 7 at some future date, it would help the Yonge line.
If the plan for the SRT subway had remain to Sheppard, you could open that in phases as well.
I still say that this DRL needs to be built to handle DD trains and be Tram-Train. You could built a connect to the RH Line around Lawrence to allow the Tram-Train to use that line. Zurich has such a line as I proposed and my idea is based on that model using short trains.
If you want to be over built now with protection for future capacity needs, you built a box station that will support 10 cars, but only complete haft of it to support 5 cars now. The rest can be built at a later date as needed.
One needs to think where the yard will be and it could be under a development.