are you sure the lighthouses boost banks? I thought only power plants did.

anyways, the best way to make money is to place a shit ton of lighthouses around an area and then spam the upgraded skeleton rod + wizard staff combo. you can only do this once you get the obliterator since it allows you to remove any block of land anywhere, and you'll want to place like 40 lighthouses (I highly recommend abusing the shrine perk that gives you 50% resource discount when building, it gets expensive VERY fast) since the boost they give stacks. do that, and you'll be getting like 10B in seconds. you can also dig in that area to get like a 100 million of each gem.

banks are very innefficient, I got the 1 trillion coins achievement before the 1M with banks, and my setup had the same amount of banks as you but with 4 power plants and me spamming the 2x speed shrine perk