Worm castings are great, you can almost grow seedlings directly in it. I don't think that's what's causing your problem. The reason I say overwatering isn't just because of the soil looking thick/wet, but also because of the small overall size for their age, curling, and spots. Those are classic signs of chronic overwatering or root problems. Aerating the water only lasts a little while after watering the plants, and if the soil is staying wet for days, that air is long gone. The humidity being up to 65% RH likely also contributes to the soil taking a long time to dry out. It might not even be that you're giving too much water, or too often, it could just mean that the soil is too thick to really dry out and provide air spaces for the roots to get oxygen.
If you're going to continue growing these plants, I would let the soil dry out quick a bit and then start giving them just a little water at a time in a little circle around the base. Once they start actually growing bigger, you can give more at a time until you're actually saturating the whole medium. You also want to see if you can keep the humidity under 50%, and you might consider getting a gentle fan and blowing it over the top of your plants to help dry out the soil and increase overall airflow.
For your next grow, if you can't get lighter soil or mix in something like perlite, you might consider starting them in a smaller container at first, like plastic party cups. It's a LOT easier to water them properly when there's just a little soil in a cup - it dries quickly and the roots always get tons of oxygen. Once the leaves reach past the edges of the cup, you could transplant them into your final 5-gallon containers at that point. Or you could start them in 5-gallon containers, but just water them more lightly and less often until they actually are growing lots of new leaves every day.
It sounds like you've got a good setup, and it's just a matter of getting everything dialed in
