Maybe the best solution is to adopt a parliamentary system of government? With the executive formally subordinated to the legislature, it can’t spend money against the legislative majority’s wishes, because if it upsets the legislature, the legislature votes it out of power (commonly with no supermajority required, just a simple majority vote in the more numerous legislative house)

Absolutely. There's a reason the US didn't replicate its own structure when it set up governments in Europe and Japan.