I have a summer 4.5 tog and a winter 9tog duvet which I alternate and always use a sheet between the cover and me. I’ve never needed anything warmer. Both (king size) go easily into my washing machine and dry on the line, before being packed away, until needed again.
They never really need much washing, because of the cover and extra sheet.
I’ve never paid a fortune for a duvet (except my first ever when goose/duck down was a fashion) and am happy to buy new if necessary.
I had the same problem with washable pillows, so I now just buy new ones every year in the January sales.