Question Using meshes instead of Unity terrain - how to handle LOD, poly count, workflow questions

Hey. I'm making a game that is very vertical in nature, with an insane amount of cliffs, overhangs, caves, etc. So I decided that I wouldn't rely on a terrain system, and handle landscapes by sculpting them as meshes in Blender. I read a few discussions on terrain vs mesh, and it seems like it's not an unheard of approach. But I'm struggling to find any information on how to handle the workflow and properly optimize it to scale well. Here are a few questions:

• How do I handle LODs? The landscapes are all continuous, how do I handle splitting them into smaller chunks? And after I do, they would create seams between the chunks at different levels of detail. Terrain systems work with tessellation, and procedural solutions can just stitch the vertices together. But with predefined custom meshes, I can't do any of that, at least not without some massive overhead that I'm trying to avoid.
• Sculpting details vs placing assets. For rocks, cliff faces and other parts of the terrain that require additional detail, I could just drag and drop premade assets. But I also have the option to sculpt them into the mesh itself. That sounds pretty great, but that would require many more polygons in my terrain meshes, which I'm not sure it's something I can afford without a good chunking/LOD system. Premade assets already come with the LODs and are separate pieces that won't cause any seams. But at the same time, they cause extra overdraw, although I'm not sure how much that would be relevant. I'm really struggling to pick an ideal workflow here and define the poly count of the meshes.
• When using in-engine terrain solutions, we can do all level design inside Unity, easily raising and lowering terrain on the fly to blend better with the assets we are placing. How could I improve my workflow if I'm doing the mesh sculpting in third party software (in my case, Blender)? And how would I even keep it non destructive with the need to chunk it all up? I'm also using a triplanar vertex painted shader, so re-exporting the meshes as brand new isn't an option.

Maybe it's worth mentioning that this is a top-down game, like an ARPG, but with a lot of verticality, so you're gonna be seeing places dozens of meters below you. To maximize culling, I believe it would be ideal to chunk the terrain into very small pieces.

Thanks a lot in advance! Cheers.