These are per-user settings. Probably the simplest is to save the settings you want in the dconf dump format, and have something in your login setup, or a systemd user service, that loads that file.
That’s essentially what the hm module does anyway, after generating the dconf dump file from the nix settings.
By the way, thanks for bumping the thread: I had tried to do this too and it hadn’t worked; looking at this example and the code to generate it, I noticed I was missing the trailing / from the entry in the custom-keybindings list