Negotiating your departure with someone angry you are leaving is foolish. Like a rough divorce, just walk away.
They shouldn’t negotiate this, they should simply unilaterally cut the cord.
What will the EU do, attack them? No.
Will they be angry at them and try to punish them economically? They already intend to.
Just declare it’s over and start running your own nation, that’s the only answer. After victory the colonies didn’t enter into negotiations with Britain as to how we would separate.
Just say “Cheer-e-o” and get the eff out.