In terms of protecting IP address of user downloading images. Can use COIL-compose and configure the ImageLoader to use a separate OkHttp client (which can be instantiated with Tor SOCKS5 proxy).
In terms of reducing mobile data, I think COIL resizes it on the fly based on the composable view size that it'll be loaded into. Still might be downloading the entire size though. IMO, imgproxy would be the best way to go.