If you haven't done it yet: Have an expert look over the [Bauleistungsbeschreibung]. You don't want there to be things in there whose implications you often don't understand as a layperson. The number of sockets and similar is in my opinion almost irrelevant. They are never sufficient in any [Bauleistungsbeschreibung], but you can upgrade that very, very far for a few thousand euros if necessary, even later on. And whether there are 20 or 30 €/sqm for tiles included doesn't matter either. But if the contractor saves on the material of the walls, you can't do anything about it later...
Best regards,
Andreas