Open world terrain approach

I know there is several threads on this topic, but unfortunatelly there are not the questions and thus answers which are important to me. I am making ProofOfConcept on UE (UE5 seems really gamechanging) and I read official terrain documentation, but unfortunatally there are rarely mentioned limits of current terrain implementation. My target platform would be UE5, (EU4 just for learning and POC).

My goal is to have 20*20km terrain with at least 1px of height map per 1m, cca 20 materials layers (each layer should be mix of several textures to prevent tiling), 3 clima types and several vegetation bioms (5-10 forest types). Terrain generating and various mask generation is not issue, I already generated everything in WM and modify outputs for any format is simple. So my questions are following:

  1. I have such setup already working in Unigine and with 20km terrain visibility it makes 150-250FPS in 1080p on 1080TI with fully tesellated terrain and vegetation (tree billboard visibility cca 5km). Is something like this possible on UE? I tried to make simple terrain (just height map, no textures) and frame rate was really bad. Maybe I am just doing something wrong?

  2. In case this is at least remotely somehow possible (250FPS is at the end not needed, I can target on reasonable 60+), what would be best approach? I would like to solve this via one landscape terrain, but if I understand limits well, one landscape could load only 8k? So loading even tiled 32k height map is out of question?

  3. So my idea was to prepare several neightbouring landscape terrains, somehow solve their borders (dont ever manually edit there, or make some tool to handle that?) and make some layered material with 5-10 layers for each landscape to have at the end cca 20 unique layers for whole terrain. Does it make sense in context of UE, or is there better and more performant way to do it?

  4. How to approach to performance optimization? Generally it make sense to bake layers into one texture, use this texture for distance and layers only for close distance. Is this something I have to solve in terrain “mega material” by myself and create baking tool for it, or is this solved automatically by UE (one would expect that, but…)?

  5. If there is any solution on the market, which can solve this, I am able to buy it (for reasonable money)

Thanks in advance for sharing your experience and insights. I would be glad for answering any of these questions or giving any hints. Currently I dont need exact instructions how to do it, but general directions.