Potassium deficiency fix? Dark spots on leaves

Royal Dwarfs:
(1x 7 weeks and 1x 9 weeks)
8L Fabric Pots
6.3Ph water/nutes
BioBizz BioGrow, Bloom and CalMag (Been following the feeding schedule since I flushed)
Growing in BioBizz Light Mix Soil
Only LST

Jamaican Pearls:
(2x 4 weeks and 1x 5 weeks)
9L Fabric Pots
6.3Ph water/nutes
Same Nutes as the dwarfs, been only feeding Bio Grow and CalMag tho. Should I start feeding Bloom since it has high K, even though theyre not in flower yet?
Growing in BioBizz Light Mix Soil
LST, Topped once and planning on lollipopping when sending into flower

Hi Ive been battling a, what I got told, Potassium deficiency for about 2 weeks now. This is my first experience growing so Im pretty hopeless at this point.
I have 2 Royal Dwarfs in late flower (one is 7 weeks and one 9), the first symptoms on both plants were yellow tips. I thought I had a Nitrogen deficiency and flushed them also slowed down on the nutrients, which only made things worse because they got even less K and C because of that. So now after the flush I realised my mistake, the flush was about 1,5 or 2 weeks ago, and am since giving them a lot of nutrients. On the younger plant the deficiency seems to have pretty much stopped or at least slowed down, most of the leaves are still green a part from the tips and the lower leaves seem to have also stopped yellowing. On the older plant over a course of 2 days things just got really bad, in the middle all leaves are still pretty nice and green but on the upper pretty much all look dead, I was told that this wont be fatal tho? The buds on the older plant also still smell and look really good, so will I be able to harvest in about 2 weeks if I just continue feeding her?

I also have a grow in the same tent of Jamaican Pearl (2 are 4 weeks old and 1 is 5 weeks all are still in veg), Im planning on sending the pearls into flower as soon as I harvest the dwarfs in about 2-3 weeks. I didnt make the mistakes I made with the other two in the beginning and just fed them a got dose of nutrients every feeding. Im also always alternating, feed, just ph water and then feed again etc.
I was really happy with how things were going and the plants seemed to be happy too and now I spotted the same problem on the lower leaves of the pearls, the most on the oldest one. The only thing you find on the net about K deficiencys is to give more nutes which didnt really seem to work with the dwarfs, I really dont want to have the pearls turn out like the dwarfs because theyve been growing so easy up to this point. There also are some weird black/dark spots on the lower leaves of my younger pearls can somebody identify? Thx

Pics: DSC 303, 302 and 304 are the pearls
DSC 298 is the older Dwarf DSC 305 the younger
and DSC 307 is the setup (3 Pearls are in the back and the two dwarfs in the front, Left is the young dwarf right is the old dwarf.) The oldest pearl is in the middle.

Are all the symptoms in the pics only K deficiency or is there more? Is just giving more nutes the only way to combat K deficiency? Is there a nutrient I should buy for the K deficiency?

If I had to take a guess, I would say it’s your ph. You should be between 6.5-7.

Your first pic; one leaf isn't enough to use as a directional point. If this is the only bad leaf, I wouldn't touch the plant.
Second pic; the plant looks very short on Mg
Third pic; the plant looks hungry and it looks short on P
Fourth pic; again if this is the only bad leaf on the plant, I wouldn't touch it.
Fifth pic; that's a 3 point leaf...one of the earliest to appear on your plant. Don't use it as a directional point...they typically die off.

If you think you're feeding enough to the point that you don't think you should be having deficiencies, I would look further into my pH and be sure it's where it needs to be. Sometimes dialing in our pH will allow the plants to uptake nutrients better, thus preventing issues. Good luck friend.

Ok thx, I get that older leaves die off but isnt 4 weeks in veg a bit early for leaves to be dying?

It is true that plants leaves die when coming to the end of their life cycle, but that doesn’t constitute a dying plant in your situation.