How difficult would it be to get all nations on this world to join the EU?

It will never happen.

Near Impossible.
Since EU by definition is a regional European based organisation. And is not open to membership to organisations outside of the region.A better question would be how difficult would it be for all countries to adhere to an EU style of rules with a similar decision making structure.The answer is extremely unlikely as those rules were born out of a shared regional WW2 destruction and a shared post WW2 Russian threat, resulting in an agreement for richer countries subsidising their neighbours. Given that a significant proportion of EU members questioning the relevance of EU today. The answer becomes much less than a 1% chance for my "better question".

impossible

Too many countries wouldn't meet the human rights & rule of law criteria

Even Poland and Hungary don't

So you think everyone wants to be ruled by a government out of Brussels Belgium? How would Americans feel knowing that their constitution would no longer be legal?

Hello, i add, every nation has appropriated union or empire, where everything in a life has specific measurements, because we discover everything from a god, e.g. an Arabians nation has "a League of Arab States" derived from "an united nations", and a Christians has "a commonwealth" derived from "an united nations"... but please verify if commonwealth is a Christians representation union or empire from an united nations, where every nation has union from an united nations,, where we interested about an united nations subsystems only, where there is union from an united nations certainly for Christians but i am not sure 100% if a commonwealth is that union, but i am sure about 90% about that, but search there is an united nations subsystem certainly for a Christians as any nation. but a soviet union and an Europe Union are an united nations subsystems but as for an outer space nations representations but not for a Christians.

very, seeing as Italy, France and Hungary want to leave and the EU have been useless on COVID.

They weren't even able to KEEP Great Britain in the EU. Greece has been on the verge of leaving too. What you don't get is that the EU is governed by unelected bureaucrats in Brussels. They can impose any regulation on the member states as well as the people they want.