Travel to Japan in summer 2021 realistic?

While no one knows when Germans will be able to freely visit Japan, July 2021 is an entire year away. There's no reason to cancel a trip that you can't even book yet (You can't book hotels until 3-6 months in advance, so there's really no rush). You should be able to make a tentative schedule.

I think almost definitely your trip will not be affected.

If you listen to the recent press conferences by the government officials, you will realize that Japan is actively trying to reopen the economy, and there's a clear priority in revitalizing the economy.
Starting from July 10th, event capacity cap will be raised to 5000, and that limit will be completely eliminated by August.
Furthermore, there is a Go-To campaign for domestic travel starting in August.
https://biz-journal.jp/2020/07/post_166698.html

At this pace, I'd think Japan will likely be fully reopened by end of the year at the latest.
With domestic tourism plan already made, I see the international tourism plan/timeline to be made very soon. While coronavirus future trend may be unpredictable, I don't see it affecting the government's reopening schedule, which is based on set timelines.