I've had the game a while, but recently upgraded with the expansions and still getting to grips with a lot of the changes. In particular, I'm having some extreme trouble in a Difficulty 7 game versus a Fortress Baron and Shield Ninny. I went with two defense-oriented AIs since I figured pursuing the Spire campaign would give me enough mass waves my first time through it. I've been pretty careful with gate raiding and keeping the possible ingress points to my territory low, while keeping a handful of planets on alert so nothing gets reinforced too insanely.
I'm nearing the end at around 15 hours, but all of a sudden these defensive AIs have started becoming insanely offensive (outside of the spire event/exogalactic waves). I don't have any core/home or adjacent planets on alert, only a handful of mark II planets. Yet apparently over the 15 hours I've been playing, the AI homeworlds have been reinforcing themselves and have begun sending out waves of thousands of Core/markV units in carriers. I've got some scouts on the AI home commands, and despite not being on alert and with plenty of adjacent-to-human sectors to reinforce, they're filling up with thousands of core units and I'm getting notifications of carriers every 15 minutes or so. Its been a tug of war, and as I've struggled to keep a defensive line against these carriers, I've ended up taking worlds I probably shouldn't have, ending up with an AI progress around 600. Regular waves and the exogalactic waves aren't a problem at all.
I'm beginning to think I'm too outmatched at this point- defeating thousands of core ships decimates a good portion of my standing forces, which then take time to rebuild while more carriers are built up. Is there something I'm doing thats prompting the core worlds to massively reinforce without having them on alert? Are there any strategies to minimise the threat carriers pose?