@OsirisTerje Sorry for the delay, I was off sick and couldn't concentrate enough to do reviews.
I also like code generation, but I do think your second option is nicer.
I think we can remove a lot of conditionals in the other projects by utilizing the AssemblyConfiguration set in the Directroy.Build.props

You added various unnecessary null suppression operators (!).
All of them are unnecessary as NUnit.Analyzer does the suppressing for us as it knows those cannot be null.

Still some issues.

I raised one issue in the build.cake script.
It was mentioned originally in my first review but got lost in the further discussions.
I pushed up a commit for that.

One thing though is that if I build this locally it build version: 4.0.0-alpha.88
So maybe the code relies on annotated tags instead of normal ones.
git describe returns 4.0.0-alpha-244-ga604a39ca

I do see confusing tags in the repository some have a v prefix others don't

4.0.0-alpha
4.1.0
4.1.0-alpha.0
v4.0.0
v4.0.0-beta.1
v4.0.1

Note that the CI build doesn't fetch any tags and creates version 0.0.0.0:

I added a commit to fetch the tags to see what it does. You can revert this as it isn't needed for CI builds only for publications.
It seems to create 4.2.0 versions:

MinVer must pick up the 4.1.0 tag and create the next release.
There are various properties for MinVer we can use to tailor this.