Rather than raw letter frequency, more powerful is frequency of digraphs, and that surely does not appear random in that context, instead spiking with common digraphs such as IE, EI, UN, NU.

Also going for longer cribs such as NUL, EINS, ZWEI, DREI etc proves generally fruitful. There is also a story about the crib HEILHITLER toward the end of a message being particularly useful for code breakers but I don't know if that is genuine or contemporary disinformation to cover true code breaking capabilities, akin to the story of carrots being good for darksight[1] as cover for improved radar capability.

[1] https://www.smithsonianmag.com/arts-culture/a-wwii-propagand...