We won’t be adding custom durations for these options however what you can do is use the following commands to adjust whatever seconds that you would like. You will then see User Defined in the power settings. Shutdown or reboot will save these settings.
Sleep Shutdown
printf 1234 >/run/muos/global/settings/power/shutdown
Idle Display
printf 1234 >/run/muos/global/settings/power/idle_display
Idle Sleep
printf 1234 >/run/muos/global/settings/power/idle_sleep
Swap out 1234 to however many seconds you would like. You were almost close with how you wanted to modify! Check out how the internal system works by checking out this FAQ page, Internal Variable Initialisation