I have experience with two systems. The first is Fox Farms soils (Happy Frog and Ocean Forest), supplemented with GH Flora Trio synthetic liquid nutes. When I was running like that, I would either feed or transplant into fresh amended soil when my runoff fell below 1000 ppms, aka 2.0 EC. I rarely fed in veg, because I was transplanting frequently, using Happy Frog. Right before I'd flip to flower, I'd transplant into the final pots, using Ocean Forest, which is heavily amended. Then my first "real" feeding might be three or four weeks into flower. Depending on how the plants were doing, I might feed one or two more times, before cutting off base nutes two weeks before chop. A single feed would be 600 ppms at the ratio recommended by GH for that week, watering slowly to runoff, which might require between a quarter and a third of the pot volume.
I switched to 100% coco coir and dry nutes a couple years ago. Because the media doesn't come with nutes in it, and it drains better/faster than soil mixes, I feed the whole way, using ebb & flow in seedling and Blumats in veg and flower. Despite being media-based, it's a very different style from amended soil mixes.
I realize the following is a hot take and not widely held: the nute lines all work. The synthetic lines are all made from the same agricultural commodity chemicals. The ratios vary, which means the lines give us different levels of control. I like dry nutes to save money, and I like a "part A" that doesn't have any nitrogen in it. That's meant Jack's or Masterblend for me, although there are plenty of other options which will work great. I tend to avoid lines that are obvious money grabs. I'm not going to name names, because I'm not trying to start a fight.
