Well, you are starting in the wrong place
If you're on your own and needing to simplify the navigation, I would have gone to the Hook instead and out through Arnhem so you could follow the A3 to Nuremburg and then the A9 to Munich.
Brussels - Aachen - Cologne gets you back on that route. The A3 is the M1 of Germany and it's 2 lanes in places and can get very slow, especially between Frankfurt and Wurzburg.
If you want a more scenic route, that's not too slow, you could try Brussels - Namur - Luxemburg and then either of two routes.
A31 to Metz - A4 (toll) to north of Strasbourg - A35 - across the Rhein - A5 to Karlsruhe and then the A8 to Munich.
Luxembourg - Trier - A1 - A62 - A6 to Ludwigshafen then down under Mannheim and Hockenheim and the A5 south to Karlsruhe and the A8 to Munich. Probably not best on your own 'cos there's lots of route changes.
I would expect the Luxembourg - Metz - Strasbourg route to have the least traffic, if only because the French are too mean to pay tolls
This is the way I would go.
When we go to Landsberg am Lech (our twin town west of Munich) we go across the channel to Calais and use the French autoroute A26 - A4 to Strasbourg and pay the toll just for the relaxation of having a drive on a motorway with very little traffic.