So it's possible I just never noticed the message noting minibuffer-next/previous-completion but those are pretty cool.

But I think I was more thinking of tabbing in the opposite direction for minibuffer-complete; the situation I'm most often finding myself with such an abundance of options is when I'm exploring available functions and just looking; I rarely want to go one-by-one in such a scenario.

…but what I just discovered is that functionality is already provided (I was confused when I saw that "backtab" was already assigned in minibuffer-local-completion-map); it just doesn't seem to be there when auto-completing in Eval: (which just responds with " is undefined"); which is how I'm most often looking at other available functions. That must be why I thought it never worked.

Welp, I appreciate the info., regardless; I definitely learned a bit more than I knew before. And it's good to know backtab will actually work in M-x, if I ever find I need it.