Upper Flint on Grouse has good steps now, it's steep. But the bottom is bare so it doesn't meet your snow requirement. Do you need steep snow right from the start?
I'm probably going to start with the Grind for now so don't mind if it's not snow the whole way. How's it looking right now? Where does the snow start and will spikes suffice or have you been taking crampons? I'm a little bit hesitant to take the Flint & Feather by myself at this time of year since I've only been up once before in summer and I'd say it sees almost nobody this time of year.
I second the cabin lake trail, this meets your needs perfectly. If the ski slopes are closed, just go straight up them to Mount Strachan on the other side, or even Christmas Gully in the later season when there are no avalanche concerns.
Similar to the point above I'll probably stick to the more well trodden, well marked routes for this. But out of curiosity, where is Christmas Gully? Looking a t Gaia here I see two prominent gullies on the Strachan side, one climber's left of the chair and one further away to climber's right which joins up with the "Collins" run. Is it one of those?