Keep one list and add to it whenever any of five things happen. A customer asks a question. You explain something for the second time. Somebody hesitates before buying and you find out why. A customer nearly chose somebody else and tells you what almost decided it. Or you notice something about your market that a person outside it would not know. Each of those is a piece, and the list grows faster than you can publish once you are actually capturing.
The reason this beats brainstorming is that these ideas arrive with evidence attached. You know a real person wanted the answer, in their own words, which tells you both the topic and the phrasing. Ideas generated in a planning session are guesses about what somebody might want, and they produce the material that gets written and read by nobody.
Add one source outside your own inbox: the questions people type into search about your category. Free tools will show related questions and the phrasing used, and reading those alongside your own list usually reveals two or three things customers wanted to know and never asked you directly, which is the most useful category available.
Batch the capture and the writing separately, because they use different attention. Collecting takes seconds and happens throughout the week. Writing takes a sitting and happens on a schedule. Businesses that try to do both at once produce nothing, because the moment a question arrives is never a moment you can spend an hour writing.
Then accept that the list is finite and that this is fine. Twelve genuinely useful pieces answering questions people actually ask will outperform fifty produced to fill a calendar, and when the list runs short that is a signal to talk to more customers rather than to invent topics.
Keep the list somewhere you can reach in five seconds, because friction is what kills this. A note on your phone works better than a well organised document you have to open a laptop to update, since the moment a customer asks something useful is rarely a moment you are at your desk. The capture has to be effortless or it does not happen, and a list of forty rough notes is far more valuable than a beautifully structured one containing six.