Help! What is this?! Upper leaves are turning yellow / brown in late flowering.

This is definitely a multi-issue problem that started some time ago. Unfortunately I don't think it can be solved before harvest. You will be starting your next grow a little more informed with DWC. Its a learning experience. Keep the PH between 5.8 and 6.2, your deficiencies will always show at the lower end, in DWC this is rare. Anything over 1000ppm will generally be overfeeding. This is not light burn. It could be a light issue combined with lockout from a continued wrong PH balance. At the end of flowering your leaves are going to turn yellow, or a different color, but in a more evenly fashion. I'd cut back on nutrients across the board, down to 700ppm or so, keep the PH at 6 to 6.2, and take her through harvest. She is not in bad shape, she actually looks pretty good. Best of luck in your harvest!

Too much feed imo. Lower your ppm and make sure your VPD is on point for bloom as well because that could also be leading to crispy leaves (dry environment).

i'm not arguing about whether it is optimal or not, but that pH in the range you use does not cause such damage. i run at 6.0-6.2 range. never had a canopy look like that since i used FF trio on my first grow - and ff trio is trash for the reason their instructions suck and you need some long-term trial and error to work somethign out that doesn't burn the shit out of your canopy. this is a common issue with many fertilization brands and their suggestions/recommendations.

The wrong PH you are in dwc so do not go more than 6.2 keep it 5.6-6.1

Senescence exacerbating an imbalance... my 2 cents. Definitely not light burn. It is the same deep into canopy and not just at the top. Would need to know how it progressed to better diagnose. Maybe Ca-issues, which could be lockout or an actualy ca deficiency of the molecule present. LEaves look a bit crispy in general indicating being overfed in more than 1 way i bet. 1000 ppm is too much for 99% of plants even if supplementing CO2. Maybe, with extra atmospheric co2 and lab-precise contrpl over other environmental variables you might be able to feed plants at that high of a dose to match an accelerated growth rate. It's not foie gras. Your light and atmospheric CO2 are limiting factors on growth rate. You want your nutrient uptake to match that growth rate needs, nothing more and nothing less. Your Caclium is too low and your Mg is sky high. Shoot for 100+ calcium and 50-75 Mg. 1-1-2 NPK ratio and 4-2-1 K-Ca-Mg ratio is the ballpark you want to start in. then, adjust. i guarantee the issues will pale in comparison to what you see here... tiny adjustments needed vs this multi-issue problem of your current plants. Start around a total concentration of 1.2-1.5EC for ambient CO2 conditions and a loosely controled environment at least avoiding extremes. You may need to go higher and that would only be a testament to how well you are controlling the environment and the lights giving "max" DLI relative to those variables. (35-40 DLI for ambient co2)