Plants are 22 days above ground. In big-box potting soil (Kellogg organic potting soil) + 30% perlite. Under HLG 600Rspec at lowest dimmer setting, 24” height, lux ~7200 (20/4). Temps low 70s; humidity 65-73 or so. Watered 5 days ago with 1/2tsp Recharge per gallon (pH 6.4, filtered tap water TDS 150). Have been checking dryness of upper soil/weight of bags and watered 500ml each again today, just before pic. Lower leaves seem to be yellowing, and some feel a bit dry (not quite crunchy). Are the plants telling me I waited too long to water? Time to start nutes? (I have

trio, and read to use 1/4 strength starting around 3-4 weeks?). Something else wrong?
I will be spacing these plants out to a second 4x4 tent once (if!) they get bigger. Any suggestions appreciated as this is my first grow.
I was in your situation 2 weeks ago, unfortunately I wasn't able to save them, they did recover for a bit but the damage was done.
I am certainly not the person with experience to give you advise but I'll tell you what I think I've learned from the help here.
The soil was one issue (possibly), the idea was that there is a huge demand of soil in the last year,
which possibly causes the soil not to be at desired quality. I switched to coco now to avoid that problem.
Perhaps you might have your "pot lifting" weight wrong, 5 days without watering seems like a long time.
so possibly the bottom of the pot dried out, feeding 500ml to dry soil will only wet the top layer so the bottom roots stay dry, roots growing in dried out soil will stunt the plant. That's what I think happened to me because I was watering very little because I thought I was doing it right.
What they mean with a wet/dry is basically keeping the weight in water as 100%/50%, in general you can't let the soil dry out.
I would water till you barely see runoff and not water until 50% of the water weight is gone, perhaps you could use a scale to be sure.
In general if you take a hand of soil and squeeze it without drops of water it's time to water.
From what people told me the best you can do is repeat watering the full pot and letting it dry untill 50% of the water is gone.
I learned that keeping your soil moist is a skill and although it seems simple (and it is after some time), the internet is full of information that contradicts one another so you end up learning the hard way.
People also mentioned it could be bad seeds, do they all come from the same place?
With that said, they might be hungry but I do think they look stunted,
you said they are at day 22,
I assume they are autoflowers, around day 28 they will start to show signs of flower
At this age they would have to be bigger and you should be starting LST in the next days.
I would start new seeds at this point, even if you can save them, you will be disappointed by the results.
Its better to start over and ask questions on what you are confused about on the forum.
But again, what do I know, I was in the same situation 2 weeks ago and only started my coco grow today so my experience just comes from others on the forum, maybe some experienced people can pick up on my reply.
Anyway good luck : )