poly reduction (Low Poly) without loss of detail

Hi all,
I am a beginner of the beautiful world of RC. I need your help where to find useful documentation for my problem.
I need to map a city, and the models will have to go inside a GIS engine to navigate the environment.
The requirement is to obtain a mesh with the lowest possible number of polygons and maintain a good level of detail during 
the zoom phase. As a beginner of RC, I see fundamental questions as the correct approach 
right from 
the training phase to the new points. Should it be convenient starting from a high degree of alignment and set then a reasonably 
high decimation value? 
Also, how is the recalculation of the texture? What is the difference between the decimation factor during 
the calculation of the new calculation phase 
and the simplification tool? Using the simplification tool how should I proceed to regenerate the texture? I see that typically the texture is 
set to me 8192x8192 pixels, but I woulk like to obtain a texture of 1024x1024. In summary, I have to get a number of low polygons
and the maximum level of detail during 
the zoom phase: 
well-defined texture and sharp corners and edges.
There is one thing that I do not understand, during my tests, the reduction of polygons and the representation in RC is excellent,
but when I export, 
for example in OBJ and/or fby and amount the mesh for example in MeshLab or Blender I get a result terribly ugly and unusable, 
with totally  ruffled surfaces, 
so-called accordion. do you have any suggestion? What would be the correct steps?
What is the right texture management workflow when decimating the texture? Do I need to use tools like InstantMesh? 
However, with the latter, I see that sometimes are not easily managed. I want to obtain maximum 500,000-1000000 polygons per square 
kilometer and a resolution of 5cm/pixel. Could you please help me?
Any suggestion would be appreciated.