I don't disagree with your criticisms of the UN. They're not a perfect organization, and UN membership shouldn't be some standard of sovereignty. However, diplomats have always been able to talk whenever they want, the problem that the League of Nations and then the UN tried to address was all the backrooms conversations nations used to have that were part of the causes that lead up to the first world war. Having an international platform every nation needs to at least listen to is better than the alternative. Arguably, untill now the UN has succeeded, there hasn't been a WWIII.

It sounds a lot like you're letting Perfection be the enemy of Good Enough.

Should there be no UN because in a small proportion of situations it's actually shit and is it really realistic to have no talking shop like that at all for as long as it takes for the World to somehow get together and make a perfect entity for that?

I've given some thought to it over the years and I think that the UN still does more good than bad, even whilst being shit at some things and having no real power other than that of influencing nations in general and the World's public opinion.

Further, even if in the balance of things tearing down the UN and creating something better turned out to be the best thing to do, I don't quite see how arbitrarily kicking countries from the UN that were deemed "badly behaving" at the moment would help us create the something better since those countries would need to be there too (it would certainly help tear down the UN, just not help with the actual primary purpose of getting something better to replace it).

A talking shop for everybody using the penalty of kicking members out only ever succeeds in turning itself into an exclusive club, and at the time when the only thing that existed were such clubs (which were naturally made up of nations allied with each other) was before and at the start of WWI and lead to it and to WWII.