Regarding what you said about suddenly having more money than you could possibly spend, that happened to me too - it's influence related it seems. It happened to me right after I conquered two minor civs and, having no real use for their planets, I filled them with nothing but influence structures. About 30 turns go by and I've got some money and nothing to really spend it on (peace, large fleet, and already spending 100%), so I started rush-building the influence planets, hoping to conquer a nearby Yor world. I didn't succeed in flipping the planet, but within a handful of turns my income more than doubled, with over 1,000 per turn coming just from tourism. Seems overpowered, IMO, if it only takes TWO planets to get such a juggernaut economy, when you can also fill a half-dozen planets with *economy* structures and not get close to the same results.
I think it is quite balanced. If you can get the same result filling a planet with economic buildings, then it is just another strategy that requires filling the planet. The end result is the same, and both are based on population. It is actually quite refreshing to have more than one strategy available for the same task.