Do you mean a start order as in, which should boot first or which should you rebuild first?
If you are talking about rebuild then I think I’d be inclined to get your core server setup with XCP-ng then XO, Unifi, DC, True Command (no idea what that one does).
If you are talking boot order then you need to look if any one server requires the services provided by another. For example do any of the servers use an AD account to log on, if they do then you need the DC to be up first. Another option might be if one of the servers provides DHCP or shared storage that others need to successfully boot.
I’ve not actually looked into delayed auto start on XCP-ng, I assume it’s a thing. I should probably check and have my pfSense vm’s come up a couple of mins before everything else.