So you're using clear cups?
The moment you see roots, transplant into something NOT clear.
More drain holes, slits, whatever so you can have even moisture throughout the pot.
I actually take a 1/2" paddle bit and drill holes up the sides of the pots that I flower in so that the soil does not dry unevenly.
Also, sounds like on your next transplant you should use more soil lightener. Add more perlite to the mix.
When the pot feels light, it's time to water.
6.2-7 is a broad range and if it's less than 6.5 or more than 6.8 you can still have problems.