Usually, it is okay to repot your orchid once annually. So if the orchid is doing fine, i.e. it is healthy, then there is no need for repotting.
Miracle-Gro does not contain coir or sphagnum peat moss, so there are less chances of fungus gnats infection (as these are attracted to fungus growing on growing medium, especially peat moss). Though, it does retain water for a longer period. You can try reducing the watering quantity and not the watering frequency.
Remember, orchids require more water in summer months. So if you reduce watering, the orchid might dry out. Rain water is excellent.
I would advise you to keep this phalaenopsis separate from other plants to prevent the spread of infection. Remove those empty egg shells and clean the orchid with isoprophy alcohol. You can try pesticides/insecticides recommended for phalaenopsis. Try Neem oil.
If the infection still persists and you see more of these egg shells, then change the potting medium.
Keep me posted to the developments.