Most secret chems shouldn't be secret.

[Tarmunora]

Dangerous chems should be balanced regardless of secrecy. No more NI3, pls. That's basically it for me.

I think Vitatroll has the right idea here regarding the balance factor.
One option for making secret chems 'less secret' could be allowing you to 'reverse-engineer' samples found in artbeakers for more detailed hints, spend spacebux for a note with a clue, rare half-burnt piles of notes in Z5, etc. That way the chems won't become instantly available to everyone, but it means that you have a hope to learn the  recipes without turbonerding away in chem for hours.