I like your relatively coarse seedling mix! I bet those are some well-aerated roots.
I don't have any 300-cell plug trays, though I recently bought some 200-cell trays I plan to use some day for starting lettuce indoors. Mostly I use 72-cell trays or 128-cell trays. I WOULD use 98-cell trays, but the only ones I found were very shallow.
I have some 50-cell DEEP trays, but I use those for potting up.
