Since the siege pact buffs attacking planets and your planets durability, maybe split it?
Loyalists get entrenchement pact: provides the defensive half of the siege pact bonuses, and reduces cost and construction time for upgrading planet toughness? Maybe provide a direct bonus to planet health.
Rebels get bombardment pact: provides the offensive half of the seige pact bonus and ... i really have no idea what is doable. A bonus vs structures? Reduction to supply cost of siege frigates? More of that money for bombardment thing?
Loyalists could get a new pact (no idea for the name) giving extra tactical slots. I know the Vasari had this in 1.2 and before so its doable and would be so in theme it's not funny for a faction with turtling as a MO.
Rebels could get at tech 8 the ability to share the allied with neutrals bit, but that doesn't benefit them since they would already have it. It could boost Insurgency, but that wouldn't be a good as long as insurgency still has that issue with acting like neutrals for the peace with neutrals tech
A pact buffing solid weapons wouldn't be at all workable. Advent doesn't have them at all, outside of things a rapture mind-controlled. Ditto for one boosting missiles.