That’s quite memorable. On the Mac I have to press option-u and then a, u, or o to get an ä, ü, or ö. Optionally holding the shift key to get an uppercase vowel with the diacritic.
option-` is for grave (è)
option-i is for circumflex (é)
option-n is for eñe (ñ)
option-c is for cédille (ç)
When you called QWERTZ an abomination I simply had to upvote your comment.
I used to be pretty good at those but now I just use the feature that works (almost) everywhere in Mac OS and iPhone - press and hold the character to get a little pop-up of options: møøśę
It's slow if you have to type in another language, but in English it's fast "enough" for words like résumé.
I am on macOS as well and the diacritics I mentioned ("a → ä, `a → à) work out of the box with the "US International PC" layout. If I press ⌥a I get å with my layout.
option-` is for grave (è) option-i is for circumflex (é) option-n is for eñe (ñ) option-c is for cédille (ç)
When you called QWERTZ an abomination I simply had to upvote your comment.