What to do with all this gold?

After a certain point, money becomes virtually useless. The only thing you can spend it on effectively without using ruler points is mercenaries, and since mercenaries cost exponentially more as you go over force limits it starts to be a waste.

In EU3 the massive money sink was the ability to spend gold to gain technology. Now that EU4 has removed the ability to do so, so your gold is pretty useless.