Your content calendar isn't the problem.
Every agency I've worked with treats the content calendar like a scheduling problem. Fill the boxes, hit the dates, keep the client happy. Then Q3 arrives, three writers are behind, and the calendar quietly turns into a list of apologies.
The calendar was never the problem. The brief was.
A slot that reads "blog post: project management tools" hands the writer a coin flip. They guess at the angle, guess at the audience, guess at what the client is actually trying to rank for, and burn a day producing something the account manager rewrites anyway. Do that across six clients and you don't have a capacity problem. You have a briefing problem wearing a capacity costume.
Good briefs are boring to write and expensive to skip. Who is this for? What does it need to do? What does the client sell on the back of it? Answer those three before anyone drafts, and the piece that comes back is one you edit, not one you rescue. That is the whole difference between a calendar that holds and a calendar that slips.