Pro-mix has a 2 week "fertilizer" charge, good just for the very beginning of a baby seeds life.
I would definitely amend with dolomitic lime and perlite. Lime will nutralize the acidic rain and fertilizer, while the perlite will keep the soil from compacting in on the roots.
MG soil and extended fertilizer is not good while in bloom. Fine for veg, but it could delay or hinder bloom as the fertilizer percentages are wrong- that's why normally those extended release ferts are not recommended. I'd go the organic root, and add a bunch of organic nutes/compost/etc for the long grow season. An organic soil mix would probably work too, but not for baby's so start them elsewhere and then plants into the mix after a few weeks.
Lastly, water could very well be your biggest issue, especially in July or August, but also insects and deer are a possibility too.