> But this is a custom setup that's not portable?

Caught me.

> And how is it less tedious to have to select previously typed text all the time?

This is tedious, but I have that automated (AutoHotKey). So a single, AHK-managed hotkey does the equivalent of:

    CTRL + SHIFT + HOME
    CTRL + C
    feed into tool, paste back
    CTRL + V

So once done writing, I press that single button and it's done (CTRL+SHIFT+HOME select all text from cursor to beginning). To me, that's a better tradeoff than fiddling with compose keys, which I find to break flow. For very short text, compose key is possibly better; but again, once in AHK, it's a single shortcut. So once more than 1 compose key combination is needed, it's "worth it". But you're right: this is a custom setup and might not work for everyone.