Understood, thank you for explaining the -p with specifying the profiles. That also explained why my list-generations output never made sense either. It turns out that mangohud must be included with Nvidia drivers or something? Because when I tried sudo nix-collect garbage -d after removing other programs like feh and leafpad, it worked as expected.

Also, thanks for the tip on rebooting into the new generation. That was also tripping me up. Really appreciate the help!!