I recommend using the following two kanji (together) to write your husband's name: 鋭橿.
It is not a 100% perfect match with either the German name meaning or reading, but is quite close, as I will explain.
First, the character meanings.
The principal meanings of the first character (鋭) are "sharp; pointed", "sharp weapon", "clever; keen", "acute (as in angle)", and "elite (especially armed forces)". This is not quite the same as "weapon edge", but is closely relevant. Furthermore, the Japanese word 最新鋭, which uses this character, means "cutting edge", a term you mentioned in your last post.
The meaning of the second character (橿) is "oak" - a perfect match with what you wrote.
The most apparent meaning of the 鋭橿 name (that is, both characters treated together) would be "sharp oak", quite close to the German meanings of the name Eiko.
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Next, the character readings.
The main Chinese-derived reading of the first character is "ei", a perfect match with the first sound of the name Eiko.
The main Chinese-derived reading of the second character is "kyō" (with a long vowel sound). This is not quite the same as "kō", the second sound of the Japanese man's name Eiko, but is very close. If you look at the handful of other, different Japanese kanji which contain the same phonetic element as this character (the right-hand side parts of the character), most of them also have the reading "kyō", but some of them additionally have the reading "kō", which is closely related to the reading "kyō". So, it is not unusual at all to assign the reading "kō" to the character 橿.
If you were to show the name 鋭橿 to Japanese people and ask them how to read (pronounce) it, many would not be familiar with the second character, and would not have any idea how to read it. Those who did know it would probably guess "Eikyō" as the full (two-character) name reading, but these people would probably also find "Eikō" to be a plausible reading.
For these reasons, I recommend using this name.