Yellow Tips on new growth and discoloring. Should I flip to 12 12 after 4-5 weeks of veg?

That’s not WPM.. that appears to be residue from something sprayed on or left behind from something that was physically on the leaf. You can confirm this by just wiping it off, or better yet spray a bit of water on it, it will likely dissolve back into the water. I’ve seen a shit load of WPM growing in a basement. Never like this

you have white powdery mildew, i have a trusty grower, he says its always lost fight, some people here swear by Doctor Zymes Eliminator , never used it, don't know even if it safe in my mind, i don't care what other or word organic says, never tested Next you have either feeding problems, either ph and absorption, very hard to guess seeing only few photos ... if those clones are 18 days old, i would clean everything, everything, everything !!! alcohol sprays, cleaners , all should be used. And start again ... Honestly looks like with better understanding, you would maybe lost nothing in time ..... And would win a lot without those big issues... I thought you are trolling this site with such sad pictures and ask about flipping ... Tough dice to roll or decision to make, but act asap in any case.

Hard to help with no dairy or many details but I'd keep close eye on them as looking at the pictures they need help but more information is required sorry but happy growing

Set up: 600w LED, 50 cm from canopy on 65% strenght, I used clones that were about 20cm tall, now after 3 weeks being transplanted in the final pot (11l) they are around 55-60cm tall and didn't show any discoloring or burned Tips. I had airflow problem and little mildew but solved it with the spraying (still white marks on old leaves) and have now good airflow. Temp: from 21°Cin the night to 24°C during the day. Humidity: 50-60% Watering: I watered every 2-3 days with 300-450 ml per plant. Always 6,5pH with some liquid organic grow fertilizer 6-0-3, also well prep soil with canna professional mix and added some insectfrass and calmag in powder form. Can someone guess what is happening with some top leave Tips (only a few plants). Can I go for the 12/12 in the next few days?

First you sort problems then you flip. Thats golden rule. And you have some work to do ...