I don't know about the spots, but I do sometimes get something like that if some fertilizer dripped on a leave and dried there. Possibly even a bit of water concentrating the light? They don't look like they're having an issues. In big pots like these, I would expect to maybe have overwatering issues. You can't just dump a bunch of water in there because it will pool to the bottom of the pots and cause issues for the roots to grow down there. But until the plants get bigger and have roots there, they can't transpire much water through, meaning slow evaporation. Watering too little risks dry spots. It's a better situation with fabric than traditional pots, but watering issues is the biggest problem I've faced at this stage.
I also have been very hesitant to fertilize at this stage as I don't want to burn anything either. With the plants growing in soil, they should have enough nutes to grow another couple of weeks before feeding. Maybe feed a very light dose, but it's a point where a little may do something, doing nothing may slow things down a week, but doing just a bit more might burn and stunt them for a 2-week-recovery period. Less risky not to fertilize at all for a bit.
Those pots really look too empty. I would add at least one more inch of soil, and perhaps a layer of
diatomaceous earth on top to prevent fungus gnats. I hear a layer of perlite on top also helps control fungus gnats. You'll get gnats if you overwater and the top couple of inches of soil never really dries out.
You are doing an organic grow, which means don't use H2O2 for any kind of control as it will kill the good stuff. At this point you might start adding mycorrhizae to your water to try to build up a community of good 'stuff' in the root zone:
