Exponential heightfog is great to make the difference between your 3d and the 2d background less obvious.
You use a gigantic 360 panorama image and map it to a huge sphere around your scene (with or without a mask to hide the sky part). With a high enough quality image and some fog it can look very good. The hard part is to find the right image on google! I’m pretty sure it’s what’s these guys have done here for their high rise urban background :
https://vimeo/135624966
High rise 360 pano in HD is probably the easiest to find online! Nature is harder imo.
I started to use 2d plane trees with some fog to make my nature background…easier to setup than 3d tree and it looks better sometimes.