Take what I say with a grain of salt as I'm on my 2nd grow, but I don't think you have a serious problem. Looks like potassium deficiency or magnesium/calcium. Magnesium will show as interveinal chlorosis with a gradual lightening between the veins and then following that the tips will start to die/brown and that will make its way up the leaves. Magnesium is mobile, so it ill show likely at the lower leaves or larger fan leaves receiving the most light. Magnesium often shows first as it's a sign of a general nutrient problem, so it could be a couple things. Those spots look like a calcium deficiency. The first photo with tips turned down/dying looks like nute burn to me. It does look like new growth is showing the deficiency so perhaps it's potassium and not magnesium.
I'm doing water only so I address each pot through top watering. This isn't recommended if you're using salts as they collect in the top of the pot.
Also, if you overdo calcium, or your PH is off it can lock out other nutrients like magnesium or potassium.
I would probably do a foliar spray of Magnesium Sulfate Heptahydrate (Epsom) 1-2 TBSP per gallon, on leaves, top and bottom and stems. A very fine mist not a soaking. Do this right as lights go out to prevent burn. It's very low risk, and if this works, you know what your issue is. This stage in flower needs a lot of magnesium and calcium. You can do this every 3-4 days maybe 3 times, then I would give them a rinse so it doesn't build up. This is really low risk and very effective. My growmie just did this 3 days ago and his mag issue just went away, leaves are green again,
I would also definitely look at your PH, both res and tray. If that's an issue, adjust. That's the first thing I'd check.
Also, check your lights, they might be so high the plant cant get nutrients it needs for the energy it's drawing in, especially if it's LED. Someone suggested this to me earlier in this thread. VPD as well. Everything is all this weird connected system, one thing off and we see issues. I only know this because both of my grows have has mid grow crisis.
I'm curious what other more experienced people have to say!
Hope I'm being helpful and please remember, I'm in my first year.