Having the template dependent upon $repoDefs means the template has to be created anew for every sbt project that uses different $repoDefs. It would be better if sbtix-gen-all could be run against the template project itself. But to do that the sbtix tool would have to be an input of the project.
Alternatively, when the user project runs sbtix-gen-all the template project runs sbtix-gen-all at the same time.
If we have sbtix hosted by nixpkgs itself then this could be done easily.