Brexit should have been a near no-op. But it was a huge pain. They hadn't even planned for it or thought about it.
EU should have a pre-memeber clearing state that countries stay in and then vote every year or n years to be "in" or "out" of the EU. But part of being in the EU should be to maintain non-in versions of all policies so that members can actually negotiate and easily leave without having huge bureaucratic cost.
This would make the EU stronger and better. Currently, there are many terrible undemocratic institutions that are being used to ruin countries.