One can host their own private or semi-private forum, chat server, chan board, etc... and choose not to re-encode the images and/or permit .tar .7z .zip archives and so on. Keep the bots away with basic auth to minimize skiddie risk to platform RCE's.

It's unlikely people can move their friends to their own platform but the best way I have found is to call it a "fall-back" platform for when Discord and others are temporarily offline. Get people used to the idea that is the place to share things they do not want leaked when the big platform 3rd parties expose files. The admin can encrypt the storage and periodically zero out files and zero out empty space for privacy.

People with slightly higher opsec may choose to block mobile proprietary devices.