I've found a very easy way to get rich, unless all other clans hate you. You can sell millitary access. A friendly clan will gladly pay you over 1000 gold for 5 turns of millitary access. it's a little bit harder with indifferent or unfriendly clans but you should get at least 500 gold for 5 turns.
Selling millitary access really doesnt seem to have any drawbacks, it doesnt seem to effect the AIs willingess to attack you and the AI won't exploit it to attack unprotected provinces or anything like that.
So lets say you get an avarage of 800 for selling 5 turns to a clan. And lets say there are 5 clans willing to pay for millitary access, then you will be scooping in 4000 gold extra each 5 turns.
If you dont like doing this every 5 turns you can do it for 20 turns but it tends to give you less money. The most I've gotten out of a single clan for 20 turns is 3600 gold.
I'm playing on legendary difficulty so this should work on all difficulties.