Maybe the Claimed status is being applied too soon. NPRs could value systems enough to exploit them, but not make the Claim until they are actually invested with populations and facilities. It might be good for NPRs to value distant systems, the problem seems the premature Claim. What if NPRs could also demand systems remain neutral/free passage so they can stay linked to distant valuable systems without having to make Claims on those linking systems?
I haven't been following development changes (figured I'd just wait and see what the new Aurora will be) so I hope I'm not rehashing considerations that have already been made.
EDIT: Put another way, "realistically" diplomatic tensions would rise when actual assets/investments are threatened, which is separate from making the investments. It sounds like the original code sometimes had NPRs counting chickens before they laid any eggs.
What you have described, is fairly close to what I have implemented.
NPRs now value systems at Secondary or above based on hard assets. For Secondary, this is one of the following: Population of 5m, 25 mines, logistics facility, ideal habitable world, directly adjacent to to a Core system.
NPRs will value systems as Claimed if they are directly adjacent to a Primary system or they have high mining or terraforming potential and are adjacent to a system with a value of Secondary or higher.
NPRs will attempt to enforce their ownership of Secondary and above.
NPRs will accept foreign claims without further consideration only if they have flagged the system as neutral and they have detected player populations in the system with at least 500 EM Signature (about 10m pop with no installations, or about 4m with infrastructure) and no claimed or higher systems will be cut-off.
They will dispute foreign ownership of systems that they value as Claimed or above, depending on the variety of factors described in the Diplomacy posts.
This creates a more realistic system where NPRs are trying to enforce ownership of systems where they have assets, plus nearby systems, while leaving a band of systems around their territory where they can still interact with other races, perhaps even with small colonies from both.