Hey OP, found this thread from your post in my WIP so I thought I’d drop by and share my thoughts 
IMO stylized graphics won’t guarantee you will save time just as (semi-)realistic rendering won’t necessarily slow you down or hog your FPS; I think simplicity of workflow is key for one-dev teams with little resources, so with a simple modeling and texturing setup you increase your chances of successfully completing your project all alone.
If you’re relying on marketplace assets, realistic assets do seem more commonplace (at least for foliage and trees). On performance - Unreal’s grass system and HISM (hierarchical instanced static meshes) should make densely populated foliage achievable for small to medium maps at least. Personally I’m using HISM for managing all my flora in a complex setup where plants grow from saplings to full bloom and HISM has allowed me to reach more than 100,000 flora even with dynamically updated transforms at 60 FPS (on a somewhat high-end rig).
Whatever art style you choose, if you’re going to spend hours creating textures and complex shaders in material editor for that perfect look (a mistake I’ve made far too often), that is precious time you you could have spent on core gameplay systems or on modeling and texturing other assets for your game. For many assets even a noise map coupled with appropriate color and roughness nodes can rapidly get you going; it might not satisfy that unique art-style which you may have had in mind, but it is game ready and Unreal does a great job of making nearly everything you throw at it look good!
Best of luck with your project, we need more nature games out there!