Building a content calendar your team will actually use
Most content calendars fail because they are too complicated, not because the team is lazy.
Start with one shared calendar, visible to everyone, with three fields per post: date, channel and idea. Anything more can come later, once the habit exists.
Colour by campaign or channel, not by person. The calendar's job is to show the plan at a glance, and colour should answer 'what is this about?' in one second.
Review it weekly for fifteen minutes. What moved, what is missing, what is coming up? A calendar nobody reviews is just a decoration.
Keep an idea bank beside the calendar. When a post is cancelled, you can fill the gap in minutes instead of starting from nothing.