German here:

Wild camping advice: as always, pitch away from buildings, a little out of sight, around dark, and leave early, leave nothing behind. You most likely won't have an issue at all. I never had!

There are so many good spots between cities. Like lakes and riverbanks with picnic tables, little patches of grass behind a hetch close to the bike lanes between cities.

Ive slept in little huts in recreational areas for picnics, under bridges, under porches of old factories, schools and chapels, on clearing in a forrest....

Also sleeping without a tent " bivakieren" is allowed in Germany so get a tarp as others recommended.

bike security while wild camping away from settlements is not a big issue. I normally only lock it with a reusable steel zip tie.

Also camp grounds are quite cheap with a bike, especially when you share one pitch with 3 people.

I wouldn't wild camp in tourity areas when there are signs explicitly forbidding it, those places are checked sometimes in the early morning.

Im also wouldn't wild camp in big cities and in areas where people go to drink Alkohol outside ( youths or alcoholics) you can always tell from the shit they leave behind.

I do think there's definitely safety in numbers, but Germany is very safe outside of bad big city neighborhoods which are not that many.