I am by far not a Loire expert, but here a few thoughts. And, while I have visited castles in the Loire, I have not visited wineries there.
My sense is it is a long way from Muscadet to Sancerre. [Google maps says it is almost 400 kilometers from Nantes to Sancerre.] You may need to concentrate you visit in only certain areas.
The best rose I have ever had was a Sancerre Rose from Franscois Cotat. Made from Pinot Noir, it tasted like a richer Sancerre from SB. Most Loire roses however are made from CF. So, you cannot generalize. There are two entirely different types of Roses in the Loire (at least two) made from different grapes and in different regions (again, the Loire is very large).
The wines I have had from Coteaux du Layon and esp. Bonnezeaux and Quarts de Chaume are pretty sweet, not off-dry. Wonderful wines, but definite in the sweet category.
For me, off dry CB is very different from Riesling. Rieslings at a Kabinett or Spatlese level tend to have more acidity and are lighter bodied. Very elegant wines with lots of minerality. Demi-sec Vouvrays tend to be richer and have wonderful exotic flavors. I am more of a Riesling fan, but both are wonderful wines that age really well.
The best sparkling wines I have had from the Loire are Vouvray Pétillant from Huet.
A couple of good links for you.
Loire | France’s Most Exquisite Wine Region
A classification, vintages and impressions of Loire reds - WINE TALK - WineBerserkers Hopefully, Julian and Robert will post here. They are real experts on the region.