I definitely like your plan of combining Weingut Robert Weil and Kloster Eberbach.

If I may add another idea: You could rent a convertible (still fine in September, global warming and all) at FRA airport (e.g. SIXT has some nice ones). Then you can drive down the Rhine through wine country and stop at the wineries and abbeys.

And you could reach the Relais & Chateaux property "Johann Lafers Stromburg". Owned by famous German TV chef. Quite a cool castle with fine dining.

And as you said, Burg Schwarzenstein would be a nice R&C hotel too.

Btw, for a next Germany trip you could go a little further south, where you find the Dönnhof winery in the Nahe wine region and a lot more on two German wine roads (trip report on my blog).

Hope that helps!