@Renate: I agree, the dual slider design is a horrible UI concept.
but I found a good solution for that. it's a little app named 'Gentle Glow'.
it provides you with 4 possible, customizable Frontlight settings and allows adjustment via a brightness and and a warmthness slider. (does the later word even exists?
it is accessable through the quick settings in the control center and also allows to be placed as ball menu button.
currently it has no one-click toggle option. but the developer seems to have an open ear for suggestions and the app is open source and available via git.