Everything I read/hear says to top up carboys once off the secondary.
Of course I understand that, but how sensitive are the wines to
oxidation time-wise? I.E. 5 minutes or 5 days?

The reason I ask is I'm finding it difficult to manage the changing
volumes in the process. When you rack off sediment you now have
ullage that needs to be managed. So you top up but later you'd like
to add something else (juice for example) but you have to remove wine
to do so. This oxidation thing is not convenient.