Brief counter-questions:
Who is responsible for the execution planning?
Who is supposed to act as site manager within the meaning of the state building regulations? What exactly should he inspect "by the hour"? How many hours are you thinking of in this regard?
Of course, this hybrid form is viable if the architect agrees to it and assumes at least the service owed according to public building law...
Here is an excerpt from the model building code. Corresponding formulations can be found in every state building regulation.
§ 52 Basic Duties
In the construction, alteration, change of use, and removal of facilities, the builder and, within their sphere of influence, the other parties involved in the construction are responsible for ensuring that the public-law regulations are complied with.
§ 53 Builder
(1) 1The builder must appoint suitable parties for the preparation, supervision, and execution of a construction project that is not exempt from procedures as well as for the removal of facilities in accordance with §§ 54 to 56, unless he is himself suitable to fulfill the obligations under these regulations.
2The builder is also responsible for the applications, notifications, and evidence required under the public-law regulations.
3Before starting construction, he must notify the building supervisory authority in writing of the name of the site manager and, during construction, immediately report any change of this person. ...
§ 56 Site Manager
(1) 1The site manager must ensure that the construction measure is carried out in accordance with the public-law requirements and issue the necessary instructions for this purpose.
2Within the scope of this task, he must ensure the safe structural operation of the construction site, in particular the hazard-free interaction of the contractors' work.
3The responsibility of the contractors remains unaffected.
(2) 1The site manager must have the expertise and experience required for his task.
2If he does not have the necessary expertise in certain areas, suitable specialist site managers must be involved.
3These then replace the site manager to that extent.
4The site manager must coordinate the activities of the specialist site managers and his own activities.