This will be almost certainly be appealed to the 11th circuit, who have aleady reversed Judge Cannon twice.
The appointment of special counsels had been settled law for decades. Multiple courts, up to and including the Supreme Court, have upheld their legality in the past. Of course if it gets as far as the current SCOTUS, who knows?
What it does mean of course, is that there is absolutely no chance of this case ever getting to court before the Presidential election (not that there was really much chance of that happening anyway).