> The obvious fix would be a permanent funding stream for the UN via its own taxes - say, for example, a 0.5% tax on all individual incomes (no matter if from employment or capital gains) of all membership countries

I wouldn't appreciate this at all. The UN isn't comprised of elected officials. Many of the members represent authoritarian regimes. I don't want them collecting taxes (and enforcing it) or making decisions for and around my country.

The original goal behind the UN's creation was to maintain lines of communication between superpowers during tense periods and to create basic standards like defining basic human rights. I would like that to remain its purpose.