What problem does this solve or what need does it fill?

Allow users to insert their own startup schedules during the startup phase.

What solution would you like?

Update schedules are configurable via MainScheduleOrder, but startup schedules are not. Make startup schedules use the same kind of list of schedule labels.

What alternative(s) have you considered?

Add system sets to 1 of the current startup schedules.

Additional context

Originally asked about by effect on the Discord:

is there any way to like
insert a schedule after PostStartup but before any of the update systems?