Antiquated games literally encouraged using only one pokemon that would overlevel and one shot everything unless it was some pathetic nfe like Weedle. Rotating between 2 to 3 main pokemon generally was enough to have an advantage on nearly everything and still be ahead on levels. Only by using 6 pokemon would the overlevel issue go away.
With the automatic experience share GF can plan ahead easily and address the leveling issue. The results already showed, GF did a much better job on all the gen 8 games than ever before.