chasv77answered grow question 5 years ago IMO your temps may be too hot for one. For only a 60w light, that may indicate you are not ventilating enough.
Are your plants too close to that light as well? Was it the same distance from canopy during veg? I know growing in a box can have lots of vertical challenges.
If your internal -> external ventilation is maxed out, you could use c02 (DIY setup) to help those plants more.
I have no experience in using ppm as a health indicator of soil . I would suspect that the Vermiculite has no bearing on that. As it just inorganic material used for absorption and aeration.
I assume at 5:2 mix, your soil should drain well, so over watering is not an issue(?), unless I am missing something. FYI, Perlite is a little better additive for areation, but doesn't hold nutes and water as well.