Cactus / repoting seedlings

I started repotting of 1 year old seedlings last week.
I do 1st repotting when seedlings are 1 year old.
I have repotted 550 asterias and 100 ariocarpus and would be continued quite while.
I have not been satisfied with potting mix and have tried various ones.
The photo is Astrophytum asterias cv miracle kabuto, 25 seedlings in a 10 cm square pot.
The potting mix is crashed scoria.