Completion can be quiet when the work no longer needs you beside it to make sense.

Finishing rarely feels like finishing. Often there is no moment of triumph at all, just a flat afternoon and a sense that something must still be missing, so people keep adjusting while waiting for a feeling that is not coming. A better test is whether the thing stands up with nobody beside it explaining.

Give the current version to someone and say nothing. Watch what they understand unaided. Whatever they miss is the remaining work, and everything else is done.