What’s a good alternative to “The devil is in the details” that avoids mentioning the devil?

Getting into the nitty-gritty

would convey what you mean.

God is in the details

is a common alternative.

Devil is in the details:

The details of a matter are its most problematic aspect.

• The idiom the devil is in the details means that mistakes are usually made in the small details of a project. Usually it is a caution to pay attention to avoid failure.

• An older, and slightly more common, phrase God is in the detail means that attention paid to small things has big rewards, or that details are important.

• The devil version of the idiom is a variation on the God phrase, though the exact origin of both is uncertain.

(The Grammarist)