been growing two F1 Titan and one green gelato aut...

Are you fully saturating your pots? Are your pots at least 5 gallons? I had Autos whose leaves looked like that and kept getting worse. My soil ph was at 6.5 as well. I found out if I kept them saturated properly and didn't let them dry out to much, the issue would go away. If I let the pot dry even a little to much, the issue would start coming back again. Almost like root bound symptoms. Make sure you are giving enough water and fully saturating your pots and see if the issue goes away.

Looks a little like a phosphorus problem to me. But I could be wrong.

When someone says nematode, first thing comes to mind, what kind of nematodes? 25k named species with countless more. Around 4k of those are parasitic to plants. Nematodes for gnats are essentially to keep top 2"4 inches clear off eggs as gnats like to use moist damp top couple inches for egg laying. Possible the food source of gnats ran out? Starving nematodes resorting to damaging plant roots?. I found a concoction of isopropyl alcohol equal parts water to be effective with some cayenne pepper and a squir of dish soap, kills gnats on contact, what's left you just create arid conditions in your tent for a couple days making sure top soil dries out and stays dry until the cycle is broken and gnats are no more.