unless you have a real quantum meter, your DLI reading is probably sketchy AF. Looks liek you stripped a ton of leaves. Kinda hard to grow without solar panels, longs and storage of necessary bulding blocks, too.
could be smaller plants. could be light, but if that was the case your nodes would be stacked closer together. I don't see a ton of leaves sprouting from same spots, so there is some spacing there.
if you only did a 4 week vege, some plants won't grow much taller than that, but i think at least some portion of what you see is self-inflicted.
read up on DLI, the wiki is good enough to get the gist. I don't know "telos" so i'm not sure if they bungle it up or not in their explanation. you'd be better off using teh specs of yourlight and guesstimating what PPFD you provide on your own than using any lux-to-dli conversion app on a phone. you'd need several measurements across entire canopy at same distance and then if the app doesn't ask for area (m^2 or sq ft can be converted), it didn't actually calculate ppfd or DLI properly.
35-40 dli is about as high as you can go with ambient co2. YMMV, because several factors impact what "max" is for your local garden. Trial and error is needed. use resulting internode length to adjust intensity. take notes on what ends up working the best, and start there next time... limited adjustments from then on.
12hours vs 18 hours.. that stuff is proportional as long as you keep other factors the same. put the hours of operation you are coming from in the numerator, and that's your factor to adjust power.. 18 to 12h is 18/12 or 3/2 or 150%. This is why you often heard "you need 150% more in flower phase" .. that's only if going from 18h to 12h. if going from 20 down to 12 you need even more relatively speaking per hour (or second). Need 167% of previous power to provide same DLI (give or take.. efficacy of equipment can change at various power which would make a small impact)
hanging distance should be more about properly covering entire area of garden with good, strong light, but can also be used to make adjustments to light intensity. hopefully, it's just hung at the best distance to give you the best average umol/s over entire canopy.