I am in a similar situation like the OP described. My flight will arrive late from Germany and I have a connecting flight which already leaves at 6am

Is there a way I can stay airside and pop into one of the lounges as soon as they open?

I guess it would be ok without luggage, but I might have to check luggage and I am not sure I can go back or rather stay airside once I pick up my luggage downstairs.

Let's assume I end up landside after picking up my luggage, how soon can I go back airside when I have a 6am flight?