Hey @Marty Kearns36! Unfortunately, it's not possible for a subscriber to enter an automation partway through; they have to enter via one of the top entry points.
In this case, I would probably recommend splitting this up into multiple, smaller automations instead. Typically we recommend having one automation per 'goal' (as opposed to chaining multiple actions/goals one after another within one mega-automation).
You do have the option to set Automation A to move subscribers into Automation B once they reach the end of Automation A, so you can still chain them together in that way. That's via 'Action' > 'Move subscriber' > 'Start another automation':
We also have a guide here46 on how you can 'retire' a current version of an automation, in lieu of switching to a new one. Following that process will prevent interruptions for those subscribers already going through this automation in its current form, but allow you to send new incoming subscribers through a new version (should you choose to), instead.
I hope that helps! :)