I put this on a spare host rather than guess at it twice.
The delay runs after the start, not before it. With start-delay=20 the VM was up in about a second and the call then sat there for the rest of the twenty before returning. So the VM you set it on is already running, and what waits is whatever was going to happen next.
Ordering does exist, but it belongs to vApps: an appliance groups VMs and order decides which goes first (https://docs.xcp-ng.org/vms/vm-lifecycle#vapps). The part that surprised me is that nothing starts an appliance when the host boots. I put both test VMs in one, rebooted, and they stayed down. Autostart is a single bulk start of everything flagged for auto power on, it walks them one at a time, and it ignores order. I set order=2 on a VM and it still went first.
So a delay does hold up the VMs behind it at boot, which is probably what you were seeing, but you cannot pick which one goes first. For your NAS and seedbox I do not think there is a clean answer today. There is an open request for exactly that, a VM that waits on another VM:
https://github.com/vatesfr/xen-orchestra/issues/8433, and your case would be a good one to add to it. What I still cannot explain is why yours stopped starting at all. A delay stalling the ones behind it would explain a late start, not one that never comes up.
