DriveDX is not a login item
per se. Neither is it a LaunchAgent or LaunchDaemon. To the best of my knowledge its launch mechanism is unique. Embedded in the DriveDX application package there as an executable named DriveDXLoginItemHelper I suspect is the trigger, but I have no idea how it works or why it is launched in this unique manner.
As you've suggested, DriveDx doesn't appear in any of the locations you've mentioned, but it
does place an item in System Settings > General > Login Items > Allow in the Background when its "Launch at Login" pref box is checked.
As to the launch order, I really have no way of accurately determining the actual launch order, but there are breadcrumbs that lead me to believe it is working and has resolved a couple of minor launchtime annoyances.
As an experiment, I added Mail, Safari Technology Preview, and Music to Autostarter, and they not only didn't launch in that precise order order, but the CPU activity involved in their "simultaneous" launch was such a drag on "getting going," that I deleted them and went back to individual hotkey launches. It's been many years - since either Jaguar or Panther - since I last tried that experiment, and my recent results are the same as I remember from back then.
