Comment by eviks

Oh, I didn't see the "CTRL + SHIFT + HOME" part in the linked AHK script, only the later steps, that's definitely less tedious, though also more dangerous (wouldn't work in rich text apps with tables and pictures etc.? though maybe your tool is smart enough to deal with rich text in the clipboard) and also pollutes the undo buffer.

Agree re. the potential to break flow part of the compose keys in general (all these grammar things should be automated away), though maybe in case of needing just a single diacritic the least fiddly alternative could be something like a hotstring in AHK ",u" to "ü" (comma + a letter without spaces, which doesn't happen in regular typing or a deadkey in keyboard layout with a similar effect)