Beginning of a magnesium issue? Overwatering or poor watering? Nutrients burn?

Yes, nitrogen toxicity as well as a magnesium deficiency beginning.

Your soil should mitigate issues related to pH to some extent. I think this is more about overall balance / concentration of fertilizer or what came in the soil. I don't think it is Mg related. Mg takes about 30+ days for visible symptoms to appear after the deficiency starts inside the plant. Is this plant 30+ days old? In either case, leaf symptoms are not discrete, so if not nearly 100% confident, shoudl always wait and see how it progresses for a more confident diagnosis. Interveinal chlrosis is a symptom that can happen for a multitude of reasons. This plant isn't moving indoor and outdoors is it? If so, could be exposure to sun. Outdoor plants build up significant resistance to light outside compared to growing under an artificial light. Moving an indoor plant outside will often start with interveinal damage on top leaves - burning itself with too much photosynthesis taking place. Plants do seem a bit lush. Backing off on overall concnetration a bit may resolve your problems and should be the first step -- maybe slightly weighted toward dropping N more than other elements. Don't make too large of incremental changes and let any change you make play out as patiently as you can. Things don't change overnight, and that goes double for any buildup in the plant. It has no excretion system, so once it's in there, it's either used fast enough or continues to build up. It's good to be observant. It's good to be patient. Small adjustments and time... unless it forces a more aggressive approach.