Hi
I can see that this is a real minefield.......if you want it to be.
At the end of the day, each of us has to decide what we regard as primary and what as secondary, and be consistent in our usage of the terms, possibly making a note how we define such terms. Obviously if you are doing research for others you need to have different criteria, again the use of different terms should be defined for clarity.
I personally regard very little as primary including BMD registers and Censuses, as I’m mindful that they are liable to transcription errors and are only as good as the informant provides. Ages on death certificates are notorious for being incorrect as the information relies on a third party. But an elderly relative who has good memory recall can be surprisingly accurate; my grandmother aged 90 gave me the burial places of all her siblings, and it all checked out perfectly.

I hope this makes sense

Julia