Thanks for checking out my game! I really appreciate the feedback as well. Would this mean sourcing different assets or using an extention/setting for unity?
Sorry I'm not using Unity, so I don't know, but I'd assume there are features or extensions / addons that deal with Level Of Detail. There are some plant generation tools that are supporting automatic creation of multiple levels of detail of the same tree. Writing a script that hides hi-poly versions and shows low-poly versions of plants at a higher distance (and vice-versa) is then however not rocket science. The tricky part is the transition between versions, or to prevent pop-in artefacts. Alpha fading between versions is smooth, but causes Z-sorting issues in many engines, don't know about Unity tho.. Also, trees at the horizon that's part of the skybox may allow to use less actual trees.