You can probably do a repot again since they are small and have not grown too many roots. However, they will have started to grow new roots and you will undoubtedly break off some of those. Or you can wait a year and pot the already improved roots system in the stronger growing media and not rush things.
I have two flats of Japanese maple seedlings that are entering their third growing season. They are planted in a mix of 1-3 mm pumice and potting soil. It works well for starting seeds, but is not that good for promoting growth. I should really repot them into the coarser mix this year, but life got hectic and they are going to sit in the finer mix for another year since I have to repot some more mature trees rather than 400 seedlings. Yes, I lose a year of development, particularly of the nebari, but I am in it for the long game - 1 year out of 10 - 20 to start to look like a bonsai is not that much. BTW I will not be developing all of them into bosnai.