Thanks a lot!
1 - These scenes were set to render on 1500 samples and I used the denoiser in compositor mixed with noisy image by 50% - I actually like a little bit of a noise and that setup gives me control over how much I want it. Each shot was rendering for about 45 minutes on GPU rtx 2080 ti with E-Cycles
2 - I did use Graswald addon for the grass and the E-Cycles addon for the rendering
3 - Yes these are mostly hdris but Color Management also played a key role in overall lighting - especially exposure. Also some post production like brightening the centre also helped a lot. Here is my setup for the first image:
4 - Most vegetation models are from 3d Shaker but grass and leaves were added using the Graswald addon