So, idea/plan for going forward.
Step one is the current phase, which will trigger the summoning once their Titans die. As of now we no longer need to worry about prematurely triggering the summoning, as they're going to be starting at the end anyway. The thing is that it doesn't follow that we should use every ace we have right off the bat though. While I bet we could make a decent start on a counter-attack into the territory they hold within our city, we would really like them to have at least some of the ritual sites within 'easily' accessible areas for us, and our aces will be needed if we're going to have a chance to crack their toughest potential ritual sites.
I'd say our best bet now is to have most of our psykers doing maximum sustainable efforts denying their shit and handling Scry and Fry, with the exception of the Telekinetic choirs who would go all-out in protecting our anti-Titan forces (as they seem to be more useful against them than in anti-psyker duties this turn or next) and the second Divination Choir, which can preemptively look for ritual sites and divine weaknesses in them.
Step two would begin once the ritual starts. At that point I'd say use the largest conventional-force hammer (with special support) to clear out the city of ritual sites, while our Scry and Fry continues on the less accessible targets . This should do some real damage to their ritual without revealing any of our aces and allowing the reinforcing Void forces to at least start taking back the orbitals.
Step three would be starting to really use our aces to disrupt the outer rituals. Orbital bombardment, massed Deathstrikes (boosted by Pyromancers and aimed by Diviner), infiltration and Void shield sabotage, Berserk gas and targeted mental domination are all strong options here. It'd probably make the most sense to stagger and group these options up a bit to maximize effectiveness—you know, combine the Berserk gas and targeted mental domination to create confusion for the infiltration mission to take advantage of, and use the Deathstrike volleys after the shields are sabotaged (but hopefully before the confusion wears off). Psykers acting in direct support of one of our aces would probably be better off going all-out rather than 75% too, as their effects should be vastly magnified by the stacking of bonuses, enough so to trivialize needing to dial it back to 50% with a -10 to -15 Malus the next turn.
Step four would be the 'Oh fuck' measures as the ritual starts getting close to finishing up if it comes to it. Sending in irreplaceable heroes, throwing our Air Force at inact air defenses, massed infantry assault—just about anything would be on the table here if it has a chance to work. We do NOT want a First Circle successfully summoned, as all the losses we could potentially take with this option would likely be a best-case if Firsty shows up.