How quickly did the Ontario line get to construction? Toronto didn't have large scale projects for a long time either, and the Ontario government didn't have to give full control to a private entity to get it built. The past is not necessarily a good indication for the future.
If ARTM gets half the powers CDPQi was given by the government, projects will move much quicker.
Its really a question of political political beholdence. The Caisse was able to push through projects like the REM at full force because they're technically not democratically elected and as such have far fewer risks when it comes to electioneering.
As for Ontario, it really comes down to 2 things:
1) A lot of projects like the Ontario Line and expanded SSE and EW are based off previously studied material, and as such the design work was far easier to do compared to a new project being designed from scratch. This is why a project that is covering a lot of new ground like the YNSE isn't going as rapidly.
2) Doug Fordism. He just doesn't care, and really wants to ram through projects on his own volition. Now I know this is contradictory to what I said about government agencies, but the point I'm trying to make is pulling a Doug Ford is a very risky move. It might work out like it currently is for him, but it could also easily backfire. Doug Ford's cramming of projects is definitely rising the amount of pandering to NIMBYs that the NDP especially are committing right now.
So, maybe you're right about the ARTM and the possibility of getting more powers? But you can't really say for sure.