Much like a total deployed army cap, but a cap on total income. Is it possible?

Currently it does not exist yet in the game.

You might have a chance to have it happen through a mod in the futur, it’s definitely in the realm of possible depending on how precise your idea of it is and wheter you want to work by itself or with other mods too.

Some the mod creators seem to have already thought about similar ideas and they might make it happen in the future, but nothing is released yet.

You could also try to make it yourself, it’s not that complicated :)