there is no reason why the EU should tolerate a state of affairs whereby EU money is funnelled to the cronies of oligarchic leaders

The EPP leaders, and especially Merkel, have turned a blind eye on Orbán for too long, to keep Fidesz inside their Big Tent (and also, Hungary being a nice low cost manufacturing area just across the border).

Any discussions about Serbia joining the EU should now end.

The current accession mechanism is working as a one-way: once the process is started, there is apparently no institutional mechanism to cancel it. Turkey is still officially a candidate, even though it's been turning into Erdogan's own form of autocracy for the past six years. And the current Hungarian Commissioner, Olivér Várhelyi, nominated by Orbán, is actually in charge of EU enlargement.

The EU should also invite Viktor Orban to trigger Article 50

Again, this would be pointless. Orbán will never do that: he's having his cake and eating it. Should anyone tell him to invoke A50, he'd turn it into another propaganda opportunity: I'm defending Europe moral values against the liberal/LGBT ideology of an overreaching EU Commission.

Gradually, as the "new" EU develops, it can shed the "old" EU, Hungary included, as so much dead skin.

As tempting as the reptile analogy is, I'm afraid that actual transformations in Europe never go as smoothly (and has often implied war in the past). It would take a much bigger crisis to have a change of this magnitude, which is essentially creating an EU-bis. I expect more incremental changes - with any luck.

It's time the whole structure and membership of the EU reflected that.

Certainly is.It is not going to happen overnight though. Unless we face an even bugger crisis than what we have today.