For maintenance window just paste this line into config.php file:
'maintenance_window_start' => 1,
If you set it to 1, the maintenance window will be set between 1 and 5 am.
With the database thing just try to run the command it tells you to use. If you're using nextcloud official image just go to the Unraid terminal and paste this command:
docker exec -u 99 nextcloud php ooc db:add-missing-indices
And with the background jobs change AJAX to Cron and if you have User Scripts plugin just add a new script and paste this command:
docker exec -u 99 nextcloud php cron.php
Save and set it to run every 5 minutes with custom schedule */5 * * * *.