Word or phrase meaning unnecessary work, not "busywork"
An English idiom expressing this is
gilding the lily
Idioms gild the lily: to add unnecessary ornamentation, a special feature, etc., in an attempt to improve something that is already complete, satisfactory, or ideal:
After that wonderful meal, serving a fancy dessert would be gilding the lily.
[Dictionary.com]
There is also a single word, supererogation:
supererogation:
the act of performing more than is required by duty, obligation, or need
[M-W]
but it's used very rarely, and is impossibly formal in most situations.