If your US-ANSI keyboard supports VIA Show archive.org snapshot you can use it to create macros for the German Umlauts. All Keychron models should support this by now.
Caps Lock
for that.MO(3)
is bound to fn
# Type ö
Caps Lock, Shift-', o
# Type Ö
Caps Lock, Shift-', Shift-o
# Type ö
fn-o
# Type Ö
fn-Shift-o
# Type ä/Ä
{KC_CAPS}{KC_RSFT,KC_QUOT}a
# Type ö/Ö
{KC_CAPS}{KC_RSFT,KC_QUOT}o
# Type ü/Ü
{KC_CAPS}{KC_RSFT,KC_QUOT}u
# Type ß
{KC_CAPS}ss
# Bonus: Type €
{KC_CAPS}e=
A
) of the secondary layer