Architect - Flat-rate offer instead of HOAI for single-family house

The planning office is responsible for service phases 1 to 5. I will try to choose the later site manager (external) early on and then involve him in phases 3 and 5. Although it is not yet clear to me how to motivate him to be interested in phases 3 and 5 at all without monetary incentives,

I definitely do not see it as your responsibility to ensure the involvement of the site manager in the planning phases. From my perspective, it would be more clever to make a contract with the architect for service phases 1 to 8, which, besides the option to terminate after service phase 2, also stipulates that (and which) colleague he allows to continue after service phase 5; i.e., to make the successive service provider a subcontractor of the actual architect. If he hesitates, I would draw the appropriate conclusions from that (although I also suspect here that I think similarly on this matter as ). You probably did not want to address whether a deviation between planning and construction site in your case is the reason to accept the splitting – therefore, I limit myself here to the repeated note to the readers that, in my opinion, this is the only legitimate reason for such a split.

Then from service phase 6 to service phase 8 the new site manager takes over exclusively, and the architect also remains involved peripherally.

Just as you cannot be "a little pregnant," you cannot hold the reins just a little bit. The form of background support by the planner is too vague for me. At least I am glad that you are not crazy enough to want to do service phases 6 and 7 yourself.