One thing you could do to check if it's really displacement that is giving you problems, or something else, is to duplicate your ground mesh, hide the original and cut small patches from the copy, on which you will do further tests. Those patches should be small enough to not cause excessive RAM usage even at smallest displacement subdivision, but large enough to be clearly visible at the distance. Make sure that one patch is close to the camera and one, or few of them are further away. Switch displacement to world size, set it at small enough value that gives you good details on the closest patch and then observe results on the farthest patch - if you see good details there, then it means indeed we have an issue with displacement handling, on the other hand, if you still can't see details that you are expecting to see, then it means that the problem is likely somewhere else (too small displacement details to be resolved in the distance, not enough resolution of final output, too strong filtering, etc.).