I'll probably have to find some software to install it properly so I can use it everywhere because I hate the default emoji selector. It's got awful usability for quick entry.

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I'll probably have to find some software to install it properly so I can use it everywhere because I hate the default emoji selector. It's got awful usability for quick entry.
not quite the same thing as xcompose, but there is a built-in "text replacements" feature in System Settings -> Keyboard -> Text Replacements... where you can set a shortcut sequence like :-P to get auto replaced by other text like 😛 for common symbols/emoji you type a lot
There's also DefaultKeyBinding.dict, which, if you're willing to learn a custom grammar, has a surprising amount of power. E.x. I have an emacs-style key sequence for typing greek letters: ⌃-g, <latin analog> (defined here—originally shown to me by Lily Ballard, but looks like it originated on an old cocoa text system webpage)
There's a more sophisticated example with more types of commands used here.
(obviously still not quite the same)