It seems like wayland.windowManager.niri.settings is defined in home-manager/modules/services/window-managers/niri.nix. Is this not all you need to configure Niri through home manager, or do you need something more? Actually, looking closer at the repo, the earliest commit to niri.nix is actually from 28/07/2026, so it might just be that the home manager module is so recent that information on the internet hasn't caught up yet.
Thank you.