Space mining is pretty much the big money maker in space right now. Fly around, find valuable resources (tritium, vandium and titanium mostly, though iriil, helium, astrium and diamene have a smaller market), refine them and complete market orders. The money isn't great compared to questing/bashing, but it does give captaincy XP.
Hard incursions are best money in the game for your time, with some exceptions (Devourer and anything that likes to flee a lot). The only problem is you need at least a corvette, ideally a destroyer to survive them. So it's money making that requires an initial investment.
Other than that? Grind levels. The higher your level, the more marks you get from hunting mobs and high level quests. Ixsei Desert (the most efficient endgame farming area) can provide you with ~7k marks an hour + quest rewards. In comparison, hard incursions are about 20k per hour in idealized conditions.
I'm not sure if easy or medium incursions are decent money, it's been ages since I did those.
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