I think this tip on buildasoil site makes some relevant points on ingredients.
Subcool's Super Soil was something that gave me good results to begin with, but a closer look at the recipe and some research of my own, there's definitely a better solution. Let's breakdown The Super Soil recipe.
buildasoil.com
I know 99 out of 100 people on this site will all say to repot, repot, repot into final pot which is usually a 5 gallon plastic bucket.
As far as I am concerned that's just nonsense and totally unnecessary if you use large fabric pots and your soil is properly aerated from day one.
If you're talking growing outdoor in that greenhouse then personally I would simply plant the germinated seed right into the final fabric pot and if you have 30 gallons then use it.
I use 25 gallon for indoor.
If your soil is right, you put a couple hundred worms in each pot, plant a cover crop, then you would really have to be stupid to overwater, the soil especially if you water everytime with Yucca in the water will disperse the water evenly, the fabric pot " on a pot elevator" literally wicks water out of the pot to where you shouldn't even have a perched water table, the worms aerate the top 5 to 6" pretty thoroughly, the cover crop sucks up water and the roots keep the soil loose so overwatering in this type of a pot and doing no-till is not much of a concern.
The seedlings roots spread out fast and will fill a 30 gallon pot.
So I say plant right into the final 30 gallon pot.
If you are going to plant in small pots especially plastic and just use regular bagged soil then probably better to start in a solo cup, go to 2 gallon then 5 or whatever.
But absolutely zero need for that in a large no-till fabric pot as long as you have your soil tilth right. If your soil is mud then doesn't matter what kind of pot its in, you'll have problems.
The only time it's a good idea to start a seed in a solo cup if you're growing in 25+ gallon no-till fabric pots is if you want to grow an autoflower in that pot, even though general consensus is dont transplant autoflowers, it's just fine if you start the seed in solo cup and then very carefully transplant into the main pot when seedling is about 10 days old, by then the roots will be just touching the sides and bottom of solo cup and nothing should be disturbed at all.
Reason being is if you have a lot of worms they will disturb the hell out of the poor autoflower and usually stunt it and its game over for an autoflower.
A photo-period can grow right on through it.
An autoflower that's been given time to get a good root system wont be bothered by the worms tunneling around but when it's only a few days old the worms can knock it over and stunt it.
I am not referring to a "supersoil" here, if you go with 30 gallon pots your best bet is just a quality Clackamas Coots soil recipe like the Oly Mountain Modern mix 2 as a living organic soil and do no-till.
Add worms and mycorrhazae and plant a cover crop and the soil does 98% of the work for you.