Rhys
Your observation are not dissimilar to mine.
I would imagine temperature to be the biggest factor with my seedings.
Cool nights and warm days going from cosy propagator to more open greenhouse/ outside.
Checks growth. I plant under fleece but I am sure my greenhouse fluctuates to much and the damage is all ready done.
I am interested to know if all your later sowing do better. I’ve never sown so early and wonder if I need to adjust my setup to accommodate early sowings or just wait until after March 10.
Leanto greenhouse south facing no shading plants moved to 10 by 8 east west facing.
Seedlings grow much better in the smaller greenhouse but my propagator is in the middle leanto.
Never used klassman deilman. Do use JI compost and have mixed results.