The US budget is ~$7T annually. There is $50B to spend deporting critical parts of our workforce. There is $1T in defense spending to ensure that we spend more than the next 9(!) militaries combined*. Et cetera.
The US spends ~$18B supporting UN programs. This is ~0.003% of the federal budget.
The point here is funds are not scarce, and in any case to the extent that one is concerned about spending, the UN spend is not the driver. The rest of your point is consistent, there's no need to use the red herring about lack of money.
* I'm old enough to remember the end of the Cold War. Americans were told that as the Soviet threat withdrew, we could expect a "peace dividend" now that we didn't need to spend so much on defense. Inflation-adjusted, we spend more now than at the peak of the Cold War.
Given the threat matrix today that includes fantasies such as "US land war against its third-largest trading partner" and the absurd "protracted war against a developing nation currently being fought to stalemate by a country smaller than California," I am not sure this increased spend makes sense. Seems like the only scenario that justifies our military spend is when a President decides to blow a wad of lives & cash in some utterly wasteful conflict.