Custom emojis is something that exists since already quite some time in the matrix ecosystem, thanks to an unofficial im.ponies.user_emotes extension to the spec. Unfortunately, element only support displaying these emojis and not sending them. But various other clients does support them: Nheko, Fluffychat, NeoChat, ...

Disclaimer: I'm one of the maintainer of NeoChat

But is it worth a million dollars a year and giving up all your internal chat logs to a company that will likely lose them when they get hacked.