There are probably other things he could declassify too. It is nuts he lets this go on while the criminals continue to mock. Enough already.
He can't declassify willy nilly. Although classifications can't be used to cover up a crime, the resistors in the deep state have not been charged with a crime (yet). There's a bit of a chicken and egg problem, but he can get some classifications reviewed by some authority (e.g. some IG) to make sure there are no legitimately classified sources and methods being revealed.
Just because we all realize Halper is a spy, doesn't mean that Halper can be revealed without some sort of review first.