I have a heat mat and a large south facing window where I germinate alot of seedlings this way you do in the video and then pick out to cell trays.
Is there any harm to the seedling if it stays in that germination tray for longer periods of time and grows larger but not too leggy? For instance yarrow seeds, its too cold here (till frosts) for me to pick out and set them in my hoophouse but they are not getting leggy, just a bit crowded.
You seem to pick these out in the cotyledon stage to transplant. Its just not warming up quick enough here for me to move into the hoophouse.