Hmm, if there is just the two languages, maybe there is split brain between X server and GNOME. I would probably avoid grp:alt_shift_toggle and configure the layout switching shortcut directly in GNOME.
And, for completeness, dconf databases also recently became configurable in NixOS so you do not need home-manager if you do not have multi-user system or all users use the same layouts (untested):
services.xserver = {
layout = "us,ru";
};
programs = {
dconf = {
profiles = {
user = {
databases = [
{
# Disallow changing the input settings in Control Center since unlike with NixOS options,
# there is no merging between databases and user-db would just replace this.
lockAll = true;
settings = {
"org/gnome/desktop/wm/keybindings" = {
switch-input-source = [ "<Alt>Tab" ];
switch-input-source-backward = [ "<Shift><Alt>Tab" ];
};
"org/gnome/desktop/input-sources" = {
sources = [
(lib.gvariant.mkTuple [
"xkb"
"us"
])
(lib.gvariant.mkTuple [
"xkb"
"ru"
])
];
};
};
}
];
};
};
};
};
Ideally, we would have option like console.useXkbConfig but it is tricky, see nixos/gnome/xkb-settings: init by jtojnar · Pull Request #257837 · NixOS/nixpkgs · GitHub.