Yeah, it's pretty easy to visit a few German fairs at any time of the year without too much effort, they are free to enter. When I was in Cologne for Phantasialand there was one right in the city, and then the Hamburg DOM was on when I was there for Heide Park / Hansa Park.
https://deutsche-volksfeste.de/volksfesttermine
Overall http://www.coast2coaster.eu/ is useful, set the coaster number to between 3 and 19 to filter out all the tiny parks, and that gives you a good sense of the major ones.
If you do Phantasialand, you can combine it with Movie Park Germany.
If you do Efteling, well the Netherlands is small enough to do Toverland and Walibi on a long weekend.
If you go to Hamburg, you can daytrip out via train to both Heide Park and Hansa Park, which are within walking distance of stations.
Personally im itching to visit Energylandia because it totally seems like an unsustainable rate of investment that will either fall in a heap or be found to be corruption, so I want to visit before that lol.