Hello,
I also use Archicad 18 for modelling the building structure / details, I find it to be a lot quicker than other 3D / Bim programs but that is probably down to me using it for 15 years. However I always export my Archicad models to 3D max and render with Vray. Although the new Archicad has the Cinerender plugin, I feel it still doesn't achieve the visual appeal that Vray has.
As with all things in the 3D world there isn't a correct way in doing things, there is just the way you are most comfortable doing whilst achieving the results you require. Your renders look very good and I agree with a lot of the comments mentioned above. Camera position and more post work will definitely help. However If you feel you are still not getting the results you want. I would recommend Archicad / Max / Vray workflow, we use this and are achieving some really good results. It is quite expensive in terms of licensing but it does allow you to use some great plugins built for 3d Max. Forest Pack Pro and Laubwerk trees are amazing and definitely improve exterior landscaping renderings.
This workflow does require a bit of management within the archicad program ( i.e. building the archicad model with materials in mind, as when your export from archicad, you export by materials, and this groups similar materials within the max model). We take it a little further and save out individual 3DS files for structures, flooring, partitions, ceiling, roofs, and landscaping. This is only because I like nice tidy max files and the flexibility to turn building objects on and off.
I hope this helps...