Bro,,you are using not one but two units yourself. You should be able to tell. But to say that the camera would be better than the 12 Pro Max would be blasphemy.

any recommendations wall/car charger and usb-c cable from third party that support warp charge?
can oppo vooc trigger warp charge?

You can use oppo or realme 65w charger. I use both oppo and oneplus charger and they give me the same speed.
Not sure if u Green GAN charger charges 65w though.
Im using a China set oneplus 9 pro and currently on oxygen os 11. No issues from playstore and volte working fine for umobile.

And no. You will still be getting oxygen os 12 in your coming update. They will merge the two eventually but not sure when. Though, oxygen os 12 does look more like color os than oxygen os.