Hi Hans,
You could easily do this by baking your maps on to a copy model with all uvs packed on one texture map.
You will need to have two versions of the merged model to this.
1. Pack the uvs on the copy model by using Cinema 4d UV edit tools. Once that is done you can delete the selection tags on your model as well as the materials.
2. Bake the original merged model onto the copy model. You can do this with zbrush or substance painter or Xnormal.
Once this is done you can use just on material to drive your photogrammetry model.
This technique is texture baking from High to low, High to high(depending on the polycount), Low to low. You can search of Youtube for more info.
I hope this helps. Cheers!