What do you shoot currently??
A lot of gear these days is so feature rich that the product of a camera displays the skill of the photographer. Not going to say gear doesn't help since we all know its a collective effort in gear/human to produce great images.
The A7iii is an all round fantastic camera. Is it perfect? nope....no camera is perfect so our pipe dream of a perfect camera just doesn't exist.
We all have different applications for gear. I would rather invest in another Canon 5dmk4 before spending money on a Sony A7Riii.
Sony, Canon, Nikon, Fuji, Panny, Olympus etc etc all have different render in their images. If you coming from a different non Sony system there are many more factors just comparing A9 and A7Riii.
If you demand 40+MP then you have no alternative but the A7Riii. If you want your typical run of the mill 24MP sensor you have the choice of the A9 or A7iii for the most current Sony line.
If your just wanting a great camera for family documentation, video for uncle Bob's BBQ party I see not reason to spend $$$ on a Sony A9. That money can go towards you know $$$$$$$$$$$$$$$ Sony lenses
Do you shoot portraits? google (images) Fuji portraits, Sony portraits and canon/nikon portraits. See how the general look you'll achieve with each Camera system.
Is your post processing skills advanced? Each camera system all have different approaches to post editing. You will see exactly what I mean if you coming from a different system like Micro 4/3, Fuji, Canon etc.
My hurdle is not spending money on the A7iii. It's the demand of Pro level Gmaster glass $$ to broaden the focal ranges in the scope of what I do in my photography.
If your not hardcore into sports or super fast requirements in AF I see no point requiring the A9. The A7iii should be able to do almost identical in performance for the average demanding user. Spend your hard earned money on native glass $$.
You are the one that must decided if Sony render is exactly what you love.