Ok first off at 20k lux is about right for those plants atm. Plants handle light intensity differently based on genetics, plant/leaf temp, VPD/transpiration rates and many other factors.
But when finding the ideal intensity one thing often missed is the duration of the light cycle and this has a huge impact on the intensity they can handle.
Eg. 24/0 with that intensity I can see it would be very possible to be light stress.
Maybe even 20/4 but under 18/6 I say that intensity is about right for a healthy plant that size.
So assuming 18/6 I'll temporarily rule out light stress.
Now bugs like mites IMO you would see a much more pronounced tacoing, Starting at the inner part (closest to the petiole) portion starting to curl in the hardest. So I'll temporarily rule out bugs.
That kinda leaves me thinking one of three things
or combination of the following. With the serrations spiking and not much curling it may just be asking for a bump up in magnesium.
Or they are getting just a bit to much of direct airflow (wind) from fans.
Humidity possibly to low. I can't remember if you stated it but I'll look back after.
I will add that your ppm of runoff is something you need to take with a grain of salt and I wouldn't read to much into it. The reason I say that is you have an organically amended soil and seeing ppm of 2000-3000 is not that uncommon. Ppm is not a good indication of the amount of available nutrients in an amended soil because there are many things that will contribute to the ppm that are not nutrients.
So in general I would always lower the light intensity when encountering an issue with your plants. Light is what drives the plant and the more you give it the higher demand is on the plant. So that means if you have an issue it will get worse faster under more light.
I'd drop the light back a bit. (Sometimes lux meters depending on the phone can be way off so unless you have tested against a known meter keep that in mind).
The node spacing will be in part contributes by genetics, light spectrum (not so much intensity unless it very low) and your day/night temperature difference.
A light closer to 3000K will grow a plant with longer internode spacing than a light with 4000K ( basically more red leaning lore spacing )
The temp difference between day and night can also impact node spacing quite a bit. Try keeping your night temps a bit warmer.
The last thing is if you are using any nutes containing PGRs?