A9 is the best current value in Sony FF?

[TonyR]

I recently bought an A7Riv to use as a my main shooter. I'd like to get a second body that can do double duty also on video when I need to do video as well. (I'm a pro wedding and portrait photographer, and I have a studio in Tunisia so I actually do a wide variety of work, more than specialists in bigger markets.

I usually don't have trouble with these types of choices. If money was no object I'd get an A1, or an A9ii (second body) and an A7siii (video), but all of these for now are out of my budget which is $1400-$2100.

Right now I feel the A7iii used is so over priced. I can find an A9 for about $2100 used, and the A7iii is about $1600, basically the same as the A7Riii, which also seems like a better option.

Is there a reason not to buy a used A9 or an A7riii to use as a second body and for video? 

Am I missing something? Other suggestions? I know A9 doesn't have s-log.

[ph_LDA]

if you want to shoot video, a7iii is the best within the ones you picked, also good for low light stills