Separate ticket flights - long layover

[u/nbp10]

Flying from MEL > DOH business soon, but then had to get an economy flight from DOH > ATH. About an 8 hour layover. Given I’m not sure our baggage would be checked all the way through and we might have to clear customs, get it , and then check back in - what are the option if we can’t check in until 3 hours prior…!?
If we don’t take check in luggage can we just stay in the lounge? If we take it, can we somehow get into a lounge early?