I'm using greetd 0.10.3 on nixos-unstable. For a few weeks, my wayland 
window managers crash when i run them like this (NixOS [1]):

   services.greetd = {
     enable = true;
     settings = {
       initial_session = {
         # command = "${pkgs.bashInteractive}/bin/bash -c 'sleep 3 && 
hyprland'";
         command = "${pkgs.hyprland}/bin/hyprland";
         user = "sperber";
       };
       default_session = {
         command = ''${pkgs.greetd.tuigreet}/bin/tuigreet --user-menu'';
       };
     };
   };

The hyprland log [2] mentions that no GPU is available during the 
initial launch. With the configuration above, tuigreet comes up as a 
fallback and it works from then on. The same happens with niri window 
manager.

I figured out that it works if i delay the launch of hyprland/niri by 
about 3 seconds (the commented out line in my config). I found this 
thread [3] about making lightdm wait for the GPU but didn't find a 
similar option for agreety.

Can i make agreety wait for the GPU to be available?

[1] 
https://discourse.nixos.org/t/greetd-agreety-seems-to-launch-before-igpu-is-ready/52118
[2] https://github.com/hyprwm/Hyprland/issues/7547
[3] 
https://discourse.nixos.org/t/x-attempts-to-start-before-graphics-are-detected/20470/2

Your configuration is not using agreety, it's using greetd and tuigreet. 
Neither of these know about displa ydevices. However, you can use a udev 
rule and a service dependency to make the greetd service itself wait for 
a particular device to start.

The udev rule would look something like this:

         ACTION=="add", SUBSYSTEM=="drm", KERNEL=="card2", TAG+="systemd"

The greetd service dependency looks like this:

         After=systemd-user-sessions.service plymouth-quit-wait.service 
dev-dri-card2.device

This will have to match the display device you're waiting for, which 
depends on your system - you're often not waiting for *any* display 
device, as SimpleDRM is likely already available, but rather a specific 
one. Alternatively, a hyprland wrapper script or hyprland itself can 
wait for a suitable GPU.