good luck for yours kovachii , I like a lot these beautiful plants but I think for now is better for me stay away because I haven't the right condition to give them ,for me now buy it would be an expensive hazard maybe when the price are more cheap I will try .
I think that people overemphasizing the importance of the soil mix where these plants are find, I think is important but the most very important is some parametric like temp.humidity water and wind .for example a beech tree don't grow in the lowlands firstly for a question of temperature and not for soil questions .I think when kovachii seeds fall on the ground what they find above goes well, the seed stay there and there grow of course ,nothing else ,isn't true that they want CA because grow on the limestone cliffs or another things ,they grow there because the mother plants stay there .but of course if the experience suggest that they doing well adding oyster ,why not adding this element in the mix?but is for me a marginal aspect ,some plants are confined in some place not only because there find best condition but also for random reason ,otherwise why if in the world there is a lots of place like Perù habitat only there grow kovachii plant?there is thousand of case like this in the world ,plant that grow only there.anyways I don't want say that the soil isn't important of course it is !
helpful is "experiment and test" and good advice (that I can not give you for now
maybe later),attempt ,attempt and attempt I have fear that you get the good informations principally only from own experience,anyways try for you is a low risk because you are a good grower and you live in advantaged place with good climate.
so try grow kovachii like you want and experiment all you want maybe get ready to lose some money and do not think at Andes landscape.in Italy say:se sono rose fioriranno.(per me son rose e fioriranno perchè sei bravo) good luck.
Anna