Got my 9 today and sadly it just aint it.
You might want to try a 9 Pro. But you said that had the same issue so I'll recommend something different, install Graphene OS and if it's software it might go away. Graphene neither voids warranty nor weakens your device's security, and it runs the same Google apps. It does have one drawback which is Google Pay cannot be used.
I am surprised though that the issue remains on the 9 Pro. I would think it's hardware related. The 9 still uses a non-LTPO panel which is just a terrible cost-saving measure at the $800 launch price and for the same reason (and others) I always recommend against the iPhone 15 as well.
A non-LTPO panel was fine for the Pixel 8 as it was available for around 500 bucks with various retailer promotions (I was able to get the price down further with Best Buy's trade in with a ridiculously old phone that I couldn't have sold for any amount if I tried). But at $800 that display doesn't fly.
This issue was also present in the Pixel 7 on launch, which was eventually fixed with Android 14.
The P7 is as bad now as it ever was, which is to say, very bad. It stutters like crazy even after a factory reset and I don't think it's because of the 60Hz as even an old iPhone SE doesn't stutter on 60Hz. It might be related to the garbage panel though, if you had the P7 you'll know that from even just a slight angle you'll see rainbow colors. I don't know how but Google really managed to make the display worse than that of a P4/P5.