For a plant of that size, you should top feed it with diluted nutrient solution. Seeing that the roots are just breaking out of the net pot, I would raise the water level so that it's just below the net pot. Plants react to gravity so they know where to grow and there's no reason keep them away from the surface of the res - you're not going to train the plant to get its roots into the nutrient feed. Just raise the water level so it can take up the chemicals it needs to process the food it's generating.
My res is a SuperPonics res which comes with a top feed mechanism. It works very well by running the top feed pump twice a day for 10 minutes. That's the manufacturer spec and that frequency and duration ensures that the hydration is soaked. After using top feed for a few grows, I removed the plumbing and went to manual top feed (I only grow a few plants, at most, so the extra pump + tubing was a PITA).
Instead of running the top feed system, I just pour a few hundred ML of nutrient solution over the hyrdoton a few times a day. That's for a 4" net pot, though. Since you've got a much larger mass of hydroton, you'll need to use a lot more nute water. Volume is πR2h so when you go from a 4" net pot to a 6" (?) is a signficant change.
Re. the doser - it's handy but it also means that you don't know if pH is changing unless you monitor the run time of the pump. The new model of the doser is WiFi capable so the app should show that info. If you've got an older model, as I do, you'll have to check it by scrolling through the menu system. It's a bit of a PITA, but it's the only way to understand what's happening in the res and understanding what's happening is very important.
I'm very much a technologist (I've worked a VP Technology during my career) but understanding
why things are happening is as important as knowing
what is happening. So yeh, the doser is nice but I believe in crawl, walk, run.
Re. EC = 200 and PPM = 300
EC is measured in units and tenths of units and maxes out at, perhaps, 3. PPM is calculated from EC.
This link provides good info. I measure in PPM and report in PPM but, seeing that PPM/TDS are often confused and confusing, I use the annotation "350/500" to indicate that it's the 500 scale. To be unambiguous, I'd use EC. Minor point.
Re. your light - AC Infinity products are, overall, very good products and I'm a big fan of theirs, with the exception of their lights. Their lights have a hot spot in the center so it can be tricky to get a good light cast on your plant. That's down the road, a bit, though. At the moment, I see a seedling that's leggy and that's a sign that they're not getting enough light.
I would strongly recommend that you get either a PAR meter (I use an Apogee) or Uni-T lux meter. In most cases, the lux meter will suffice because, in practical terms, it is as useful as a PAR meter. I've attached a document I wrote that will help you convert from lux to µmol. For your light, I'd go with a conversion factor of 0.015.
About the AC Infinity light - it's a "value product" in that it's inexpensive but not a very powerful light. Check my grow journals for light levels but, for my most recent grow, my 9 day old plants were at 360µmol running 24/0 for a DLI of 31mols.
Re. the leggy seedling — strongly recommend that you add silicon to your nutrient mix. I used Botanicaire's Silica Blast but have switched to making my own. It should be the first ingredient when you mix nutrients. The reason to use it is that it strengthens the walls of the plant stems and is very effective in warding off mold. Check for postings by "
@farside05" for instructions on how to make your own, if you're so inclined.
Your set up is a lot like mine and you might find it handy to spend some time going through my grow journals. They're "data rich" and written to help other avoid the f*ckups that I've run into over the years. My most recent grow turned out to be a bust but, if you check out the journals from early 2023 and older, there's good info there.