seeds don't need any
nutrients at all before they have begun growing their first set of real leaves. Then when starting to use nutrients on them, I wouldn't use any more than 1/4 strength. Your pH is fine, honestly I never really check pH for seedlings myself (I'm just lazy that way). But if your lettuce sprouts are still falling over, a few things come to mind. First what is the temperature? Lettuce is a cool weather plant, and may have trouble with temps above 80F. The second is even though you are careful not to over water the
growing medium, their roots may still not be getting enough oxygen. You might try using a different growing medium, something that dosen't hold nearly as much moisture, and a lot more air like perlite or vermiculite. Lastly the relative humidity could be an issue. I don't seem to really have too much trouble, but I don't grow a lot of lettuce and I know it's more important for seedlings to have good humidity.
They should be able to start growing a couple pair of leaves without any nutrients. Even when starting to use nutrients a real weak
nutrient solution is better, and a pH between 5.5 and 6.5 is in range. So honestly if the roots are getting enough oxygen, the temperature are in the mid to upper 70's, their getting enough light (and it looks like enough light for seedlings), and relative humidity is good, I'm stumped.