Regarding the MiracleGro Potting Mix. If you're using the one I linked to, I've seen people use it. But, they usually have problems because it's so strong (and, it's time-release synthetic nutrients, I believe. People try to flush it, which you do if a plant's overfed. That just burns the plants more because it releases more of the polymerized nutrients. It's not organic sources that the plant can take up as needed.). I haven't seen anyone say they use it on a continued basis. (It's more like your situation. Saw it at the store. Decided to use it. You're having much better results. Probably because you haven't tried to feed.).
However, I have seen people who say they use MiracleGro Organic Choice (0.10-0.05-0.05) as their soil of choice, have good results, etc. That's about half as strong as the "Potting Mix" version. (Be aware, there is a "potting soil" version of that, which is stronger. I think that's for outdoors.). OC might contain organic sources of nutrients, not time-release synthetics. You might have to feed that a little, at some point. But, it shouldn't be "too much" (which I think you're running into right now). If you wanted to feed more and rely on the soil less, Black Gold organic potting mix is (0.05-0.00-0.00). Personally, I like to feed every watering and be more in control of what the plant's getting. But, I've never used that product. (I would add perlite to any of this. Lots of perlite. At least 30%. Maybe 60. I like the soil to dry in 2-3 days when the plant has grown into the container. That's one problem you have. That plant's in a fairly over-sized container. That makes it challenging to keep it uniformly moisturized, but not too wet too long. It's hard to balance that when the container is large, the roots haven't filled it yet.).
The soil I use is documented
here (<<link). That document links to the nutrients I use. I've used it for 5-6 years. Created it on a whim, without much thought. It's worked really well for me (and the nutrients I use). I'm experimenting with hardware-store soils now. I'd like to replace the Pro-Mix part of my soil with something else (something that's not a specialty-store item, more readily available). But, it works really well. The specialty store aspect is just a minor thing. I'd just like to simplify it a little.
I think your plants look great for your first grow, and using a somewhat problematic potting mix. (If you buy perlite, get Vigaro or something. MiracleGro's perlite has 0.10-0.10-0.10 added theirs. (It's like they can't refrain from nuting anything they touch. I was thinking the other day, the cafeteria at MG headquarters probably has an NPK value for every item on the menu. "Meat Loaf: 8-2-5." Who would add nutrients to *perlite*. It's like they wreck everything they touch.