Sounds like a good plan to me.

I do it all the time as well. Good thing about potting mix, is that the plants zap a good chunk of the nutrients out of it quickly when in the pots (bags). Since you didn't use crap ferts like say, Miracle-Gro, salt buildups should be minimal. That said, overdosing on NPK isn't likely.

Keep in mind that it will break down rather quickly due to all the organic matter - come Spring, it may be near where you are now and look like you did nothing due to compaction and microorganisms breaking it down. If you keep after it, the low spots will fill in nicely in time and you have the benefit of great recycled soil! If you run out of low spots, spread the remainder in a thin even layer over your lawn about a 1/4-1/2 inch thick in thin areas - will give a nice organic boost!

Miracle-Gro is fine. We're talking potting medium from a pot put onto a lawn. Whatever fertilizer salts are in the pot will dissipate quickly, not that they would be harmful to begin with.

People get WAY too fussy about this stuff.