If it works for the login screen, I would expect it to do so for the user as well. What specifically have you tried?
I have assumed that it changes the scaling-factor key of org/gnome/desktop/interface schema but it looks like the scaling is per display in the Settings app so it might actually do something else.
I do not have a HiDPI screen so I am not sure. Try running dconf watch / command and then try to change it in the Settings app.
Looking at the source code of the Settings app, it forwards the configuration to Mutter
Possibly it changes the scaling factor in ~/.config/monitors.xml, while the scaling-factor GSettings setting is just the default. Maybe try deleting ~/.config/monitors.xml and re log in.
Or you could copy the monitors.xml into your profile configuration and let home-manager manage it declaratively:
home-manager.users.fomm = { lib, ... }: {
home.file.".config/monitors.xml".source = ./monitors.xml;
};
I would not recommend that. Looking at the source, it mostly just messes with fonts.