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.
Requirement
- Set the compose key of your desktop environment. I use 
Caps Lockfor that. - The momentary layer key like 
MO(3)is bound tofn 
Default behavior
# Type ö
Caps Lock, Shift-', o
# Type Ö
Caps Lock, Shift-', Shift-o
Custom behavior
# Type ö
fn-o
# Type Ö
fn-Shift-o
Macros
# 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=
Setup
- Create macros
 - Bind them on the non-umlaut key (Example: 
A) of the secondary layer