Guix allows you to easily write own package definitions which are treated as part of the whole system. If you submit them and they are accepted, they simply become part of Guix.

> why must said repo prevent me from installing older versions,

Precisely this is a case that Guix solves extremely well: One can start environments with specific versions of packages, like in Pyhon's virtualenv.

And by the way, the complexity is caused by the issue that many many components need to fit together. That does not happen by chance, it is a massive amount of work to make it fit in a manageable way, and check that it works. That's what distributions do.