You’ve got eleven detritivores (plus any hitchhikers from the rock), and only two small algae eaters. 12-15 small snails and 2 larger ones is not too much for that tank, in fact the recommendation used to be one snail and 1.5 hermit crabs per gallon. I think that’s overkill, 14-17 snails and maybe 10 blue leg or 5 scarlet hermit crabs would be about ideal for a tank this size (maybe a couple less hermits since you have nassarius snails). If your parameters are otherwise in check (though you didn’t answer my other questions), it’s your cuc that is anemic. I’m also not sure why you’re reluctant to add more live rock, it would only make your tank more stable (and probably more aesthetically pleasing).
Just for comparison, I set up a 28 gallon Nanocube (basically the same as a biocube) on Jan 20, I’ve got four fish, a coral banded shrimp, corals, 4 scarlet hermits, two turbos, 4 margaritas, 4 cerith, 4 astreas, and 4 nassarius, and my tank is basically pristine. Part of that is staying on top of water chemistry, but the other part is having an adequate CuC.
It’s your tank, so do whatever you want (I mean, you obviously do, you have a tang in a 32 gallon), but you asked for advice, I just told you what I think is appropriate as far as CuCs go based on my experience.