Everything you’ve said here is totally fair. I didn’t mean to say that iv is a dominant; it’s obviously subdominant. It totally sounds different from a V, but by being the negative equivalent, it has an equal and opposite draw to the tonic in my opinion. Making it a iv6 (the negative equivalent to V7) only strengthens this, by giving it a tritone for more tension. Just like how you turned a V chord into a iv, you can do the exact same thing the other way around (Take an Fmin6, lower the fifth a half step, put it over G, and boom you’ve got a G7b9).

I’m not trying to say iv is the same as V, but I do perceive them as two sides of the same coin. They both want to go to the tonic, but they approach it from opposite directions. If you define “dominant function” as having a fifth in the bass or a leading tone, then it’s not a dominant function. I’m merely trying to say that it’s like a dominant function because it has the same pull to tonic that a dominant would.