Thank you everyone for your ideas, advice and discussions.
It's been a long journey, but I think Ihave reached a destination that will work - and if it doesn't I should appreciatee the issues and alternatives. It feels as if we have delved in every nook and cranny and levered boulders to see what might be lurking beneath.
The system isn't one I ever imagined, and there are aspects that aren't ideal. I haven't eradicated dependence on databases, but all my own data sits free and accessible and dependence is reduced to tags and links in one program only. Workflow should be smooth and simple as against a rather Heath Robinson set of steps for a pure document approach, and it copes with fragments and snippets that aren't so easily coped with in pure documents. And its accessible on all devices and OSs - though that's through a web base.
The workflow is incredibly simple. I spend most of my time in a text program - any text program - writing, and spend most of my playing and working out time in Trello. Using specialist programs (mindmaps, spreadsheets etc) whenever I want.
If I'm reading and want to make notes or take quotes, I do that in a text program. I can switch around as often as I want. If I'm thinking of ideas or jobs in the garden, then they can all go in too. New line for every new think. Periodically, probably when I'm gettingfed up with doing that, I will copy them all and paste into Trello as separate new cards; that bit should take less than a minute.
The Trello housework is about linking and tagging and then I can move cards around and play and think, maybe, of new ideas to be written as above.
Pure writing again done in a text program - possibly the same one but maybe not. A Trello card will contain links to the document.
The system works as well for researching medication, planning holidays or evaluating anti-flood products as it does for writing and research.
I have the same continuous workflow whatever I'm trying to do.
All my data, including the content of the notes, is held in documents independent of any database.
The only things held only on databases are the tags and links and the structured relationships between cards. I'm hoping that Trello is big enough and stable enough to outlast my need for it. Plan B is to use CSV exports to switch to anouther appication or find another way of managing.
I will report back on progress and snags.
EDIT addition - just to point out that I will do initial tagging when I first write note, which should make recovery a bit easier if Trello fails.