I have a 4.5 for summer, and a microfiber 10 tog for winter, and sheet for hot weather. I also have an old-fashioned quilt that I use as a throw.
In hot weather, I use the sheet. If it is a bit cooler, then I put the quilt over.
In warm weather, I use the summer duvet.
In warm weather that is a bit nippy round the edges, I put the quilt over the foot part folded in half, and pull it up if cold or fold it into quarter, if hot.
When it goes cold, I put the winter duvet on with the quilt on the feet, folding it up and down as needed.
When it is very cold, I replace the quilt with the summer duvet and fold it up and down as needed.
Flexible