Xcompose only offers sequences started with the "compose" key (whatever it is mapped to) and resulting in a single X key-symbol, so to type an emoji needing multiple uniciode codepoints that way, you'd have to enter multiple "compose sequences". Whatever you describe here is not simple Xcompose. AFAIK, GTK offers entering unicode codepoints in a similar way ...
Of course you can do a lot of things with "input methods" (like popping up some "picker" only when wanted). But then you'll depend on X clients being aware of them in some way, e.g. by implementing the XIM protocol. Lots of applications have that (even xterm), so it might be what you prefer and that's fine. My method with faked key press events is definitely hacky, but will work with any X client, that's the point of it ;)
Of course you can do a lot of things with "input methods" (like popping up some "picker" only when wanted). But then you'll depend on X clients being aware of them in some way, e.g. by implementing the XIM protocol. Lots of applications have that (even xterm), so it might be what you prefer and that's fine. My method with faked key press events is definitely hacky, but will work with any X client, that's the point of it ;)