First possible issue: using landscape splines.
That’s a no-no. Honestly don’t know whey they haven’t deleted them yet (probably they think that it would cause complications with compatibility or they are idiotic enough to use them on fortnite which kind of makes sense 
)
Use regular splines, then apply the sculpting to the landscape via bluetility function call, in editor.
Secondarily:
No one really uses landscapes other than “youtubers" and similarly rediculous people (including the fortnite team!)
Use an external (dcc) editor to create things with. Houdini by SFX is decent at it.
Blender works too in a pinch.
If you really must endure the mess that landacape has to offer, then stack yourself up for success. You will need to eventually remove it and use meshes. Plan ahead and don’t use features that will be “gone” like lakes, rivers, grass etc.
Look up how you bake landscape impostors and get familiar with the pipeline as well.
For best results you will need to export the final sized landacape and split it up at least 100 drawcalls each to allow some more apt occlusion and reduction of memory stored tris count.
(note: if using nanite you may not need to cut it up, it depends. I have 0 idea how they implemented it. I know epic enough that I can just say “it’s wrong" and be 100% correct. It won’t truly stream only what you see like better engines have done for 20 years plus and will likely retain the tris in ram + add more to it by computing on top of it! 
)
Anyway, That should give you some ideas of what to expect.
I’d say that if you really must work in engine with landscapes, do so in 4.18 to use them.
4.25/27 or later versions if any may come to exist - in order to bake and replace with meshes.
Once baked out, give some 5.something engine a try - levarage nanite. Load up a full scene by importing it or converting a copy of the project and benchmark it (then go back to 4.x because the engine is just all around worse XD).
