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For KFW it is enough. Many people I know only take the test around the completion/handover date.

Nothing must go wrong, though. 1.5 will probably be achieved better.

It is hard to change anything, e.g. a window not completely sealed, because everything is installed and finished.
You will undercut the 1.5 and that is enough.

As haydee writes, the handover is obviously sufficient. However, it makes more sense to do it before the interior finishing. If you now get drafts through the fittings and the ceiling spotlights on the upper floor, you will hardly tear down the tiles and drywall again.

That was only meant as an illustration.
You do the blower door test (apart from the requirement when using KfW funds) to check whether the vapor barrier is properly connected and airtight, whether everything is tightly plastered, whether house entrances and sockets are sealed, etc.
Therefore, you do the test when the windows are installed, everything is plastered, and the roof (from the inside) is sealed. Because now you can still make improvements.
In your condition, you can no longer make improvements.
Accordingly, that was what my first comment meant, you don’t need the test anymore. (Unless you are using KfW funds)