Week 5 yellowing, N deficiency or natural fade?

The lower drop at week 5 is pretty normal yeah, but the mid-canopy pale leaves are the part worth watching. That's the tell for me. Healthy senescence works its way up gradually and the leaves that go yellow still look "alive" if that makes sense, they yellow clean. Pale and washed out mid-canopy usually means something is actually short, not that the plant is finishing gracefully.

That said, before I touched nitrogen I'd be looking hard at the calcium situation. Cal-nitrate is your N and Ca source in that mix, and if uptake is off even slightly, Ca tends to show first as that general pale washed look people misread as N deficiency. Your pH at 6.4 is fine, but EC "seems reasonable" is doing a lot of work in your post. What's the actual number?

From my own experience, mid-flower pale that moves fast is almost never just natural fade. I've chased phenos that want more feed mid-flower than I expected, especially resinous ones. I'd rather bump the feed a touch and see if the new growth holds its color over the next 5-7 days than just let it ride and end up with a plant that coasted into the finish on fumes. You can still taper off in the last couple weeks. Missing the window on the other side is harder to recover.