How to define a new highest header while keeping the others the same?

[PgBiel]

@Alberto

I had the idea that if I could somehow move every header one spot down in the hierarchy, I could redefine the first one (=) as my personalized chapter header.

You can use #set heading(offset: 1) to “bump” all headings by one level, as follows:

#let chapter = heading.with(level: 1, numbering: n => [Chapter #n.])  // <--- override level calculation
#set heading(offset: 1)  // <--- heading level by default is depth (amount of '=') + offset (default zero)
#set heading(numbering: (chapter, ..n) => numbering("1.", ..n))  // <--- omit chapter number from heading number

#outline()

#chapter[Origins]

= Title

== Subtitle

=== Subsubtitle

#chapter[Epilogue]

= Title