I think you have a very good idea.

Indeed, the Cookbook is aimed for very small things (e.g. one or two
procedures to do something common) so it has a different focus.

Here's an existing list of Scheme tutorials:
<https://schemers.org/Documents/#intro-texts>. Probably most of them
only cover the basic language, not applications, so that would be a
third focus.

A master list of all these resources would be useful :) Should we add
(at least some of) them to the Awesome list?

>     For that project to succeed, it must be extremely painless to run
>     and test
>     the application itself. That is it must have Continuous Integration
>     and a
>     standard way to run the code.

These are very good points.

>     Guix requires sudo to install it. Also, it is not portable across
>     schemes
>     or kernels. I read there is a GNU Hurd port and maybe xbsd port.

Guix is not currently in the official FreeBSD ports (freshports.org) or
OpenBSD ports (openports.se). I think there's a work-in-progress NetBSD
port (http://pkgsrc.se/wip/guix).

> I keep coming back to this issue of build-systems and package manager.

Have you tried Akku (https://akkuscm.org/)? It supports both R6RS and
R7RS packages and IMHO Göran has done a good job making it
user-friendly. He also actively tries to support as many Scheme
implementations as he can: <https://github.com/weinholt/scheme-docker>.
So that would be one option.