You take great care of your plant, so I doubt this is anything but a natural process. But try what you have planned and see. You are almost at 6 weeks, you may have a faster finisher but either way your buds are super developing and look great so if it was me personally I would try to arrest the yellowing but if you try too hard to re-green the plant it might do more harm than good.
I also just lost lower/mid fan leaves, my ph was fine, new growth was super green.. The sativas they don't like too much foliage from videos I watch. They seem to naturally lollipop with stuff on the bottom. Just a theory, the way you have topped your plant and spread the hormones to produce tops your bottom is now mid point on your many colas, which with these strains would normally be one big 6-7 foot branch when mature. It may be natural. Anyhow you are an experienced grower with great results, you already know not to throw too many different things at the plant at once. You are trying a different mix of fertilizer from foxfarm than you have been from what I read, so maybe it is as you think just a food issue.