I have some tomato and pepper transplants and zucchini, cucumbers, beans, brussels sprouts, carrots, and greens that I will be starting from seed on a Boston (Zone 6b) black rubber full sun roof deck. I will be using a stacking tower for the greens and 15-25 gallon generic Smart Pots for everything else. We're installing DIY drip irrigation, but I'm still concerned with moisture retention, weight, and ease of obtaining potting mix given the harsh environment and current situation. Although I know it would be cheaper to mix my own totally from scratch, I don't want to go to many different places to pick things up, I don't have a great container to mix in, and the convenience of bags that I can personally carry up 3 flights of stairs may outweigh (ha!) the cost benefits of buying in bulk. I will need 50 cubic feet in total, and can manage a saturated weight of the standard 120 pounds per square foot, though lighter would of course be better.

Does anyone have suggestions on what I should do? This is my first vegetable garden, so I'm totally starting from scratch. I could just buy outdoor container potting mix as-is, but I feel like it might be better to lighten it up with peat, coconut coir, composted mulch, and/or perlite. Also, do you think it would be worth adding a couple of inches of true mulch on top for moisture retention and cooling? Thanks for any advice!