Why am I of the opinion that an EC of 0.8 - 1.0 can be good? Because as I mentioned earlier, im working with clones and growing with no substrate on every system, with every fertilizer manufactor, every light and water supply is slightly different. My Water EC is between 0.2/0.3, so a Batch with 0.8/1.0 is a big Thing. I'm not far away from what you recommend ...
Unfortunately I can not tell you why your plants with EC 0.2 - 0.4 have problems. For this I would have to know all the circumstances and you know that yourself
I use the two components Hy-Pro fertilizer in my recirculating soil tank and i give to 100l water 250ml each A/B compund. If I get the clones, they are already set to an EC 1.0. As you can also see in my report, the EC sinks. So the plants really need more. If you look into my report, you can see vital plants. With Hydro / Aero i do not need to increase the EC unnecessarily, I almost always work with the same EC up to harvest.
What I personally noticed is that under purple LED light the plants are more stressed than under warm white COBs. In addition there is the cultivation without substrate. Most of the strains have not been that bitchy so far, but sometimes I have a strain that does not want to grow like the others. What is your Groundwater EC/PH?