[...] because only Workstation uses gnome-initial-setup.

Just two random (and stupid?) ideas:

1- Replace Anaconda in Fedora Workstation with a screen that asks your language and another that  asks if you want to "Erase everything and install Fedora" or "Install Fedora alongside existing OS". All the rest handled by GNOME Initial Setup.


2- Split anaconda into "Anaconda Installer" (language + partitioning) and "Anaconda Initial Setup" (everything else) and apply this to all variations/spins (except "Anaconda Initial Setup" to Fedora Workstation).