We've noticed a few yellow leaves. Do you think buying Cal-Mag and a pH test kit would help? Any advice?"

I don't see calcium or Mg issues, so i don't think a ca-mg product would do much for you. Looks like it is paling from bottom up on the plant, so i'd assume it needs a bit moreN. Also, looks over defoliated on the one plant, which wille exacerbate any sort of nutrient imbalance, if you cut off a portion of what was stored. Training is find, but leave the leaves, until the canopy is full underneath otherwise you are only causing a net-negative effect with less photosynthesis potenetial from the canopy. I'm not a fan of the pH probes. They end up costing more over time and a hassle to store properly than a cheap pack of good pH test strips that resolve down to .5, which is good enough. The 4-color strips are easy to delineate the colors, too. avoid 2-color or "1.0" pH resolution, obviously. I bought a 100-pack 3 years ago and still have 80 left, lol. 5 USD... 1.33/year and i still have 80% remaining. You don't need .1 resolution accuracy for this. The shade of color can still inform a very educated guess as to whether it's closer to 5.5, 6.0, or 6.5 pH, etc. In soil, obviously 6.5 is a good target. soilless/hydro i'd go 6.0 because it gives leeway both directions. anythign near or below 5.5 is going to lockout Ca, first, then other stuff if it continues to drift downward.