A peace treaty is too much, You should start with a cease fire, with robust terms for handling minor breaches. Try to tone down the war to something like the situation between Pakistan and India.
My rough draft:
Confeds recognize the existance and authority of the Rebel government in certain regions of space, Rebels recognize existance and authority of confed government in certain other regions of space.
Rebel and confed navies agree to a Cease Fire. Unarmed ships are permitted to move freely through confed and rebel space. Armed vessels, or vessels carrying munitions may be detained by rebels when in rebel space, and by confeds when in confed space. Any vessel may be subject to search according to the local government laws. {insert rules on search and seizure}
If the Cease fire is breached, the attacked party can respond with all necessary force to repel the attack, but may not pursue the attackers beyond their space. 24 hours after the attacking force is repulsed, this Cease fire is automatically reestablished.
Confed and rebel agree not to launch "deep" attacks (eg, no confed raids at palshife, no rebels attacks at earth). {insert list of restricted planets}
Covert activity is OK if nobody is killed. Large scale sabotage, big lies, and propaganda are OK as well. Kidnapping and assassination of high-ranking government officials is forbidden. {insert list of protected persons, ranks and offices}
Engagements between warships in Independent systems are not considered a breach of this Cease Fire, even if ships are lost, thousands killed, etc. If a warship in the engagement was escorting a convoy of non-combatants, the non-combatants can be seized by the victors but not killed or destroyed unless they attempt to flee {insert rules for seizing non-combatants in independent systems}.
Each party will make a good faith effort to recover survivors from the other's ships after an engagement, even if the engagement was a breach of the Cease Fire. Any survivors recovered must be treated as POWs. All non-commissioned officers must be ransomed or traded according to the following schedule {price list for various ranks}. Officers and government officials can be held indefinately. If an officer is ransomed, the price must be individually negotiated.
{details about how POWs can be treated. Interrogation, torture, rape, starvation permitted, brainwashing, "accidents" forbidden.}
This should provide a enough room for military gains to be made while protecting large areas of space and large segments of the population from the war.
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