Your first picture of the seedling looked fine. If anything some over-watering can cause your leaves to droop like that but no sign of nute burn on your leaves so you were right to just wait and not try to adjust anything in your grow.
When you grow from seed, once the seedlings break the surface and have access to light then a lot starts going on under the surface in the way of root growth, or in your case, inside the rockwool cube.
Nothing much will seem to happen above the surface while your root structure continues to establish itself but then once it has grown enough to start sucking up the nutes, then you will notice substantial growth above ground, especially once your roots start growing out of the rockwool cube, through the hydroton, and into the nute solution below the netpots.
You will want to get your pH up from 5.3 to at least 5.6, preferably closer to 5.8 if you can in the DWC.
Perhaps your seedlings improved once you moved them into the DWC because of better pH/temps/nute solution in the main res. What are the temps/ph/ppms in the DWC?
Good luck and grow on bro!