Most importantly, +1 for the dormancy points. I completely agree.
In terms of fertilizer, if you want to stay organic, I'd recommend bat guano. Regardless of its particular N-P-K, it'll be low in nutrients compared to fish emulsion. I throw 'em whatever I have on hand and they seem to enjoy it. I mix it in the soil upon potting, and treat them with a tea anywhere from 2-5 times throughout the grow season.
For trace minerals I give them a dilute seaweed emulsion tea. Again, maybe a few times throughout the grow season. At least in my hands, the cacti don't seem to burn it up fast enough to cause thick parts at feeding and thin parts during the intermediate times. They grow pretty evenly once they get going.
I've personally used time-release All Purpose Osmocote a couple times throughout the season with great success. Same with Plant Tone All Purpose.
Lastly, you can avoid a couple "big" feedings by continuously feeding with a dilute fermented feed with every watering. This works great for all plants on which I've tried it, btw. A paint bucket with an airstone is the basic setup. Wrap up some lawn clippings, maybe a pinch of guano or something, and some healthy soil (as an innoculant...probably not needed but what the heck) in an old shirt or some cheesecloth. Put it in the bucket along with 4ish gallons of water and let the bubbler go for 24ish hours with occasional stirring. Dilute to taste (I use a cup per gallon and feed at every watering), and your done.
It makes a great foliar feed as well. I know of people that even take a small leak in the bucket. That's just plain fun...
All of these can be used separately, or in conjunction with each other. Just add less of one thing if you're going to try another approach in the same season. My collection gets the scraps of food that are on hand from other more picky houseplants. Maybe it's not 100% ideal for cacti, but hell, my guys seem to be doing just fine.