This way we have direct access to each generation’s activation script
and we should be fine with (3).

WDYT?

> Yes, I think that makes sense.  I need to learn more about GuixSD's
> activation mechanism, but it makes sense that a procedural definition
> (e.g., like in NixOS's switch-to-configuration script) is equivalent to
> a declarative definition.  I think a declarative definition of services
> would be fine.  Concretely speaking, is there a specific format that is
> convenient to use for such a "declarative definition of services"?

I would suggest an sexp as discussed at:

  https://lists.gnu.org/archive/html/guix-devel/2016-06/msg00173.html

> What about the grub installation?

The ‘parameters’ file should be enough, as you wrote.

HTH!

Ludo’.