The size of the textures affects optimization and calculations and it is extremely hardware dependent.
A 1060 will fart out with a 2k texture applied and work fine with a 1k texture applied.
Whereas a 4090 or 3090 won’t even bat an eye at the texture size (GB of Vram being availiable to them).
Check this for some real optimization info - landscape specific.
https://www.intel.com/content/www/us/en/developer/articles/training/unreal-engine-4-optimization-tutorial-part-1.html
Based on your terrain sizes I think you should just try a full (single) 8km^2 tile as an export/import into Houdini.
It will probably work fine, and allow you to import another couple tiles after its processed/set up as a displaced mesh.
Remeber that you need to simplify the mesh after import so that you have as little amount of tris as possible…
Overall, the considerations you are making are good to be made, but usually should be reserved for areas exceeding the 50km^2 - I think.