I second Komoot. It’s actually a German company. It’s quite popular and there are a lot of ratings for the different streets. This gives you a good idea about popular routes.
An other advise: In Germany it’s super easy to take your bike on trains. Some even have dedicated bike-cars. You might have to buy a ticket for a couple of EUR. So a weekend in the Alps is just a few hours away for you.
If you want some hills, ride down south. This area is pretty nice: https://www.nationalpark-eifel.de/en/
Regarding bikepacking. One of the most famous routes is riding along the Rhine (flat and a lot of dedicated cycling paths). Probably it’s also popular because it’s populated with vinyards. Head over to Cologne, ride down south via Bonn, Koblenz (and get a new Canyon or just visit their store, it’s quite nice) and continue to Mainz! Good times!