You wrote :In my view, this is not enough, since all of the cacti are getting half of the amount of sunlight they could be getting (since only about half of each plant's surface is exposed during the day)

BINGO

FL,

Ml2u answered it succinctly but completely. These need to be outside / inside a solarium which gets sun all day (if they had to be, which in your case they don't). If there's something about your environment which prohibits having Feros outside all day in their growth season (now), isn't there a way to combat it? Your present growing conditions are fairly well unsuitable for this genus of cactus.
{{gwi:514755}}

I'd love to grow them outside, and I surely can, but I haven't been able to successfully combat all, or at least some of the pests. I guess our Wisconsin summers would just naturally be a purgatory for barrel cacti. What the arctic is to cold, Wisconsin is like to these pests. We are ridden with humidity, lakes, and bogs... :)

Artificial lighting is unfortunately just too expensive to rig up, but that would work, I assume.

I'm getting the feeling that trying to grow healthy Ferocacti and Echinocacti indoors, in Wisconsin, without supplemental lighting is just a little too much to bite off. Oh well, I guess my choice of cacti are poor companions with the Wisconsin environment. After all, they are meant to be grown in the Southwest! :)

Thank you for your help.

This is FL signing off. :)