I wonder: could we look to sports, the NFL, for a solution: There the tension is between getting the call right and keeping the game flowing. Could we give the president a couple get-out-of-jail cards per term--allow her to fire two IG's but no more? Arguably this would permit the president flexibility but not too much.
I'm afraid what will happen when the administration changes: the new president will use the Trump precedent to fire the Trump-IG's and goo-goo norms will suffer further erosion.