Hey @Andi_M8
A lot of input and a flood of information, which just develops over time.
Sometimes it doesn’t come, and sometimes it does.
That’s simply because every plant behaves differently. Some need more, and others less.
And that’s why one has it and the other doesn’t!
But you’re definitely on the right track somewhere. Because you’ve examined your water values.
I haven’t done that yet! 
But if your values are really like that, your CalMag problem doesn’t surprise me.
If I’m on the right track, the calcium should actually be much higher, and with you, it’s the other way around. And that causes the problems. A calcium-only supplement or just some liming should work wonders.
Your night temperature of 20 degrees is OK and can go down to 18 degrees at night in the last three weeks of flowering.
Your daytime temperature is a bit low, which would also be optimal in the last three weeks of flowering, and until then, I would gently raise the temperature to 26 / 27 degrees, which is even more beneficial for soil life.
And if you often only see a very slight CalMag deficiency starting from the third week of flowering, they will still finish. Because at some point in flowering, in the last two or three weeks, they will use themselves up anyway. Nature, after all.
From the pictures, you are already very well on track.
In any case, make sure more calcium gets into the soil or water, and then things will run even better. Greetings from Pflanzenpaule