If you want to have more spacing between “1.” and “Heading” in “1. Heading”, you can use a show rule to “rebuild” the heading from scratch:
#set heading(numbering: "1.")
#show heading: it => block(counter(heading).display(it.numbering) + h(2em) + it.body)
= Hello world!
== Hello.
(Update [2024-09-24]: added block() around the replaced heading’s contents to ensure block spacing around headings is preserved, see How to increase the spacing between the number and the title of a heading? - #4 by Eric)