I wouldn't bother with aging your bag of soil.
First, nitrification requires oxygen, so except in soil at the top of bag, there won't be much nitrification.
Second, nitrification removes ammonia but it adds nitrates to the soil. So now this aged soil contains nitrates and when you submerge it, you get nitrites (via nitrate respiration).
I would just put the soil in the tank and be prepared to do water changes and a little tinkering until soil settles down. Mineralizing it via Michael's method is another option, but I prefer to mineralize the soil in the tank itself.
Glad to hear that you found potting soil for cactus. Let us know how it works. In theory, I think it would be ideal for aquariums. It would have calcium and hardwater nutrients designed for cactus growing in alkaline soil, yet because cactus are slow growers, it wouldn't contain a lot of nitrogen.