Comment by akirk

I also use a QWERTY keyboard and I use a custom keyboard layout that maps alt-a to ä, alt-u to ü, alt-o to ö, alt-s to ß (plus the same for uppercase for the first 3). That works well for me without the need to post-process.

On macOS it's relatively easy to create using a tool called Ukulele (https://software.sil.org/ukelele/). You can also download my layout here: https://alex.kirk.at/USUmlaut.keylayout