The plants developed their first true leaves and began to grow the second pair. The nematode treatment in combination with the sticky traps worked as very few new fungus gnats entered the traps since the day of application.
Since nematodes need a constantly moist substrate, I watered all the pots with 2.5 liters of water on the day of the nematode application and haven't watered them since (exactly 1 week).
Tomorrow they'll get a smaller watering, about a liter per plant. This is more than I would normally give at this stage of development, but I want to prevent the substrate from getting too dry (again, nematodes can't survive in dry substrate).
All in all, promising outlook!