Quite a lot already exists. You already generate diplomatic points for diplomatic ships, treaties and POWs and lose for the reasons stated.
However, giving you informed on the amount of DPs lost or gained would give you far too much information about the NPR. You would know the xenophobia rating and the value (to the NPR) of each system. Just like in the real world, you can generally only judge other nations by their actions. You don’t know for sure what impact any of your actions had on their opinion of you.
One option might be to add an espionage result (through POW interrogation, ELINT, etc.) regarding the NPR’s general view of you.
Treaties are already periodical in the sense that they can be revoked by either party if the relationship changes. There are already trade and research treaties (and geo/grav).
There are already agreements about the ownership specific systems and you already have ‘mining rights’ anywhere, as long as no one objects. Perhaps a ‘joint ownership’ agreement could be added.
As ‘temporary ceasefire’ is effectively a reset of diplomatic points to neutral. I do think a ceasefire or armistice agreement would be a useful addition, rather than the current method of not fighting while the relationship slowly returns to normal.