Sleep/Suspend with no logged in users

I am trying to get my PC to suspend from the lightdm/slick-greeter logon screen (with all users logged out e.g. as per power on), if I have a power outage (e.g. whilst on holiday for 2 weeks) the PC will reboot and stay on but won't go into suspend/sleep.

The PC will go to a blank screen after 10 mins, but does not go into sleep.

I have done some research and have spent a good few hours (make that days) looking into this with various testing, to no avail.

The main thing I thought would work was:-

$ sudo -u lightdm gsettings list-recursively | grep sleep

org.cinnamon.SessionManager prefer-hybrid-sleep false
org.cinnamon.settings-daemon.plugins.power sleep-display-ac 1800
org.cinnamon.settings-daemon.plugins.power sleep-display-battery 1800
org.cinnamon.settings-daemon.plugins.power sleep-inactive-ac-timeout 0
org.cinnamon.settings-daemon.plugins.power sleep-inactive-ac-type 'suspend'
org.cinnamon.settings-daemon.plugins.power sleep-inactive-battery-timeout 0
org.cinnamon.settings-daemon.plugins.power sleep-inactive-battery-type 'suspend'
org.gnome.settings-daemon.plugins.power sleep-inactive-ac-timeout 900
org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type 'suspend'
org.gnome.settings-daemon.plugins.power sleep-inactive-battery-timeout 900
org.gnome.settings-daemon.plugins.power sleep-inactive-battery-type 'suspend'

$ sudo -u lightdm dbus-launch gsettings set org.cinnamon.settings-daemon.plugins.power sleep-inactive-ac-timeout 90
Authorization required, but no authorization protocol specified

(I used 90 e.g. 90 secs to test as I did not want to be waiting around for 30 mins.

$ sudo -u lightdm gsettings list-recursively | grep sleep-inactive-ac-timeout
org.cinnamon.settings-daemon.plugins.power sleep-inactive-ac-timeout 90

But no, this did not work. (I did change the battery setting too)

I've also tried

sudo nano /etc/systemd/logind.conf
...
IdleAction=suspend
IdleActionSec=5min

This did not work either.

And various other things and combinations. So I have got to the point where it is probably easier to ask.

I also am not sure where the setting is for the screen going blank after 10 mins as it is obviously not the above settings.

Could you let me know how I can change this and also it may be worth putting these settings in the "lightdm-settings" app if possible.

Thanks

System:
  Kernel: 6.8.0-57-generic arch: x86_64 bits: 64 compiler: gcc v: 13.3.0 clocksource: tsc
  Desktop: Cinnamon v: 6.2.9 tk: GTK v: 3.24.41 wm: Muffin v: 6.2.0 vt: 7 dm: LightDM v: 1.30.0
    Distro: Linux Mint 22 Wilma base: Ubuntu 24.04 noble

Start by reproducing the suspend behavior from the LightDM/slick-greeter login screen, using the referenced gsettings keys and /etc/systemd/logind.conf settings. Check whether the behavior belongs in slick-greeter or the proposed lightdm-settings app; done means suspend works with no logged-in users and the relevant setting can be changed through the appropriate interface.