Toronto is in a rare and fortunate position where there are no tunnels needed at all because there are no stranded sections of the system that stop at different downtown terminals. The central corridor through Union is sufficiently wide to be able to fit all of the existing services.
Attempting to branch the Lakeshore GO lines within Toronto adds a needless level of complication that just needs to get entangled later. I'm not going to pretend to know whether it is better to electrify one line at a time or starting from Union and proceeding outwards, but rather than have separate all-electric and all-diesel services, we should really be getting dual-mode locomotives when electrification begins. But that's a fair ways off.
To me, what's defined as a "solid project" is one that is quantifiable and gets the system closer to a built-out, reliable, electrified system. Those solid projects are things like grade separations, double tracking, and utility relocations that have tangible benefits now. Electrifying the Lakeshore line from Oakville to Whitby will be a solid project soon, and then Kitchener, Barrie, and Stouffville can be "hooked in" over time.