Think this fits in with what we know of the Rebellion and how they acted in the original trilogy to be reactive until carrying out the Endor attack where they were partially proactive.

In Episode 4, Tantive 4 was attacked and defended, The Death Star attacks and the Rebels send ships.

In Episode 5, Empire attacks Hoth

In Episode 6, Empire builds Death Star 2, Rebels martial for a strike at it.

In these books, Thrawn does various attacks, Rebels attack.

Have the rebels been ever proactive, actually rebelling? I know that in Rogue One, the rebels do be active.
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They're not the Rebellion anymore, they're the Republic, with a Fleet that can match the Imperials in open combat. And I always assumed the bases we saw were just the headquarters and they had other branches.

You misunderstand me. Having studied their artowrk, previous battles and deduced how they generally thinks, Thrawn comes to the conclusion that it would be improbable for these people to go with the predictable route rather than try to throw a curveball at him. People have paterns. If you know someone well enough you can guess when they are going to bluff. Thrawn just figured out a way to guess someone's psychology based on their art tastes and cultural background which, while I doubt would be feasible in real-life, is probable enough for star wars. As I said before, this is not a particularly wild guess nor does he risk much by being wrong.
I could buy that Thrawn knows generally how they think from artwork. I could buy him thinking 'something's wrong here, we need to investigate further' or ' they're not going to do something this straightforward, they're going to do something else' and seeking information to confirm it.

I cannot buy 'they're going to attack this specific shipyard at this point in time and I am so confident of this based on this artwork from years before this war even started that I explicitly forbid my subordinates from doing any investigating to confirm my hunch.' There's at least one other CGT in the galaxy, and Coruscant is supplied for months as per Fey'lya. Bel Ibliss believes they don't need the CGT anyway, so he could well try to get Thrawn to commit forces defending CGTs and attack, say, Ukio instead. But Thrawn guesses the exact correct plan without any investigating.

Go on; finish that thought. How would Admiral Thrawn have figured it out? The Disney Star Wars have spoiled you if you think he pulled it out of his creative spot .
Given the relatively recent major war involving Clones and Jedi, it seems like this would either be common knowledge or impossible to discover. Unless Thrawn knows more about cloning than the Clonemasters and more about the Force than the Jedi.

would say that this is directly to how much the Rebels are overselling their attack. It's a little way too much, and so naturally Thrawn doesn't buy it. Maybe the rebels could have, or should have not been so overzealous in their preparations.
They're not. That's explicit in the text. It's an entirely convincing performance of an impending attack. And Delta source has been silenced by now.

PT Era republic isn't so bad, it took decades of scheming by a precognitive telepath with enormous political and Force power to bring it down, and he had plenty of trouble doing so. Absent his meddling, for instance, Qui Gon negotiates a peaceful settlement with the TF.

CH25:

Han looks around a corner at Stormtroopers at a guardpost, and lunges across the hallway to draw fire while Chewbacca takes them out. Luke cuts through a metal door and they take on some more, then run before security arrives.

They're going to have to blow up the cloning complex, which Han is happy about because after hauling explosives all this way he'd prefer to use the damn things. That might not be enough, though, but there's a self destruct in the Throne room.

...I hope the garrison commander could turn that off somehow before he imprisoned a Jedi up there. Mara and Luke head up to investigate, while Han seals the room and starts placing explosives. 3PO calls him over for the news that Joruus C'Baoth is imprisoned up there. Comms are down, so Han has to try chase them down himself.

Inside Bilbringi, the smugglers are on schedule when the ISD fleet arrives. They all assume Karrde has sold them out and prepare to go out in a blaze of glory, until Aves notices that the fleet is arrayed to defend against Rebels.

Mara and Luke are nearing the top of the base, where there are no guards because nothing of importance is up here. You don't think the SELF DESTRUCT system might be kind of important, Mara?

They stumble across two stormtrooper guards. Two is a bit underkill given the damage C'Baoth could dfo if he escapes, but they deal with them and enter the throne room, where the Emperor has a one of a kind galaxy map. They walk up to the throne... and Joruus C'Baoth spins it around to face them. Have to hand it to him, that's a great way to make an entrance.

CH26: C'Baoth has anticipated this meeting through his precognition, and says so. He provided a distraction so they could get into the mountain, because dying in the service of the Jedi is fitting. Luyke argues about the Jedi not being Masters of the Republic, and C'Baoth calls them failures.

"The Old Republic survived a thousand generation," Mara put in. "That doesn't sound like failure to me."
I want to frame this quote. Really wish someone said it in the ST, ideally in response to "The legacy of the Jedi is failure."

C'Baoth is not interested in such petty semantics. Mara holds him at gunpoint, and he follows her directions happily. Luke decides this is his last chance to save C'Baoth, and reveals that he (Joruus) is a clone. C'Baoth is at first shocked and then amused. He rebuilt the mind of Gen Covell and thinks he can control the Clones that have such similar minds.

Mara and Luke move to leave, and C'Baoth activates a remote detonator. The Force comes back. He must have detonated the ysalamiri cages. Wow, puppet Covell was right, security isn't great in this place.

C'Baoth snatches Mara's blaster and throws Force Lightning, hitting her with one blast. Luke blocks most of the rest. He tries to help her. C'Baoth is not pleased to be ignored and introduces a figure in a Jedi robe, who advances towards Luke. When he moves into the light, Luke's own face is revealed, much like the Darkside cave on Dagobah. But this time it's a Clone, Luuke Skywalker. The Emperor found his hand and lightsabre. How? A good question, for another time.

Luuke attacks, and I may get the the two confused in the remainder of this writeup. Luke flips away, and Luuke follows him, mostly more slowly. Luke can escape, but that means leaving Mara to face Luuke and C'Baoth alone.

In the Clone chamber, Lando notices the ysalamiri exploding. R2 has found the schematic for the column supporting the chamber. The Imperials notice and start firing stun blasts rather than damage the cloning facilities. Lando and Chewbacca have no such restrictions. The Imperials are probably going to try to flank them, forcing Lando to seal the doors and lock themselves in. Chewbacca can rig something up, but the explosion is likely to be too big to survive.

Wedge comes out of hyperspace close, but not at, the Bilbringi shipyards. Ackbar has time to identify the trap before the TIEs start flying.

High in the base, Han hears someone coming, and nearly shoots Leia. She and Karrde explain that the Force is back in action. The Noghri are downstairs, so it's just them to rescue Luke. He tries not to have flashbacks to Bespin and fails.

Okay, so C'Baoth had something up his sleeve after all, restoring the Force to him in time for the duel. I'm still a bit disappointed, though. These books have been building a C'Baoth/Thrawn confrontation since book one, it it seems unlikely to happen now, as they're on different sides of the galaxy. C'Baoth still might flip his killswitch/hack on the Clones, but that's not quite the confrontation I wanted. Oh well. I could still be wrong.

And Luke is duelling Luuke. Hmm. I suspect they may be equally matched.