I think the problem Espanso is trying to solve should be addressed by GUI toolkits, like GTK, QT, etc. Otherwise, living with an authorized keylogger in our system in order to introduce unicode characters seems overkill.
I agree, though, seems like a ripe piece of low-hanging fruit that could be better integrated and safer. Even if the lower level stuff just hands it off to a dedicated tool that handles the replacement text, at least the OS/WM should be the one watching the keys.