Perfect time to transplant is mostly set by your strategy on the final volume used to flower the plants. If you rely entirely on the plants, you never stop to transplant them until the end.
Basically :
first transplant : temporary cup or seedling’s pot, the time to have a decent root ball.
second transplant : when they are ready to go truly in veg and to produce active vegetal mass.
last transplant : just before or during the switch to flowering cycle, to contain the stretch and also to give them their last home.
It’s more about your strategy and goals than a “perfect time”. If you dig the grow logs here, there are as many strategies as there are growers. Try to find something close to your will and get inspiration from it if you want to “personnalize” it to your taste.
I have found that if the substrate is too hot seedlings have a tendency to stall out. This can be temporary or permanent depending on damage to roots. It can sometimes damage the roots in a way in which they can not grow but can sustain the seedling just fine. The seedling will never grow any bigger but will remain healthy otherwise. My apologies if this was already mentioned, I skimmed most of the thread and didn’t catch it. Happy growing
Seedings were started in coco with no nutes. They see no nutes until they have first true set of leaves. Then feed is very low for first couple weeks. Definitely not a medium to hot issue. 5 seeds from 1 source did fine while 6 from another source in the exact same medium and planted at same time didn’t. They even showed much slower germination.
For me, my perfect time to transplant or up pot goes like this.
Starter medium, zero nutrients in it, after I tail my seed, either in a shot glass with plain water, or folded in a paper towel.
I like to 1/4 fill a 4" nursery pot, with wetted starter mix, poke a wee hole, lay the tailed seed in there, light cover, set on a heat (in winter) or if needed, @ 75f , as seedling grows, I back fill with starter mix.
After 6 points show, I up pot her into a 1 gallon lightly fertilized medium, that is more chunky, my fav, large chunk perlite/coco husks, with some EWC.
2 months later into 3 gallon pots, run for flower.
No perfect way IMHO. Many many ways to get started.