I'd be somewhat concerned about dumping dry sackrete into a sonotube and expecting it to cure evenly and completely. After all, when I use it on posts, it's exposed to soil from all sides...allows moisture in.

Sonotubes are wax-coated to avoid immediate deterioration from the moisture of the concrete...and keep moisture from the ground out. They don't last forever, but even exposed to weather above-ground, they are in place for months (even longer, sometimes).

If it was me, I'd use ready mix (already wetted) inside a Sonotube. Just MHO...

In a lifetime, the average driver will honk 15,250 times. My wife figures I'm due to die any day now...