Given a six-month check-in and annual review once a year for each report, you'd have a total of 60 such events a year, meaning you'd have at least one annual review or six-month check-in every single week. You'd have to keep up with the unique performance characteristics of 30 different people in order to conduct these reviews in any reasonably useful and professional way that actually encourages real growth. If there's any sort of interpersonal conflict or unexpected project complexity, this could easily burn through enough of your time that you'd have to blow off nearly 30 people in the meantime.
Nowadays I do 1-on-1s with the dev lead, the assistant, and the product owners (senior devs and architects), so only 5 people.
Still, that would mean the dev lead would need to do 25 1-on-1s, so we have a structure of peer mentorship, with two phases: a formal and an informal. The formal is to maintain quality of information gathering and fairness. The informal is to allow for rapport and flexibility in the part of the mentor. We started this recently, we'll see how it goes.
My role is to help them focus when they start to drift, shield them from politics, and most importantly give a cerimonial ratification on their ideas, if they match the values of the company.
So all 4 non developers are essentially assistants. I hope to soon be able to step out and leave the team working happily with a 10 to 1 ratio of dev to non-dev.