Hm, a lot to say possibly. Here are my notes:
- Turn up your highlight compression
- I think your biggest problem is the photoscanned assets that you are using. You made these yourself? I'd highly recommend not wasting your time making these and buy some high-quality existing assets. Mud from Poliigon is fairly cheap, looks great and will improve things a lot.
- Moving from Blender to Max is probably hurting you too. Try modelling in Max. Or using better import settings. (Sorry, I don't know much about blender).
- Tone down the contrast in the wood texture, it's quite ugly.
- Turn down the car reflections...a lot!
- The leaf textures are pretty awful. The shader needs a lot of work, and a bit help would be to have them scattered better - not a single left at the end of each branch (very unrealistic).
- I think if you fixed the ground texture, scattered the grass a bit better, compressed the highlights and got the leaves looking better that'd be a big help.
I cant say why it's taking such a long time to render, but it is very noisy which is making things worse. Perhaps you have too much reflection on your grass blades or leafs? Are you using proxy objects? This should speed up rendering I think?
Hope this helps. I also hope you get more feedback as my experience is limited. Good luck!