As long as you can't do fight missions credits are kinda hard to come by without cheesing. Unless you "liberate" a few races at the start of the game for the 5000 credit boost. Or have slave.
Time-wise, since you can move on freighters on "pirate" missions to steal additional bribe items, you can get 4-5 bribe items / mission, which is 4/500 credits + a few hundreds for ships kills + another few hundreds for rescued pilots. Highly boring but since those last about 30 seconds of "real time on strategy map", it's more or less a boring infinite source of money. I do consider this cheesing though, at the limit between bug and abuse to be exact.
Once you've got access to AFA cleaning missions, credits are too easy to get, as a single mission got me more than 25 000 credits.