> leaders often favoring national interests to what would be best for the union
If we're talking about leaders for member states, then they're doing their job right :) They're not meant to figure out what's best for the union, we have separate elections for those people who go to EU and represent the country + care about the union. The national "leaders" of the country are quite expected to put national interests above what's best for the union :)
Because of the convoluted way the EU works, those national leaders can have outsized power and even veto some decisions (Hungary, anyone?). The parliament needs to have more power if we want to make the EU more democratic.