Justifying an empire split over expansion

Hoping some creative procrastinator can assist me with this problem that has been racking my brain since I've returned to my writing.

The main story is set mostly in the country Crius. Crius was founded a good 200 years earlier by the 143rd elder council of the motherland. The elder council were essentially the same as the roman consuls and ruled the empire that governed the motherland and it's colonies. The 143rd were particularly obsessed with the idea of legacies and being immortalised in history, so they proposed the idea of another empire expansion to the east following a scout reporting that there was fertile land beyond the immediate mountainous terrain, which was the initial deterrent. The noble houses weren't really interested however, and pushed back as they saw no need for expansion and better use for the funds at home. Eventually they get their support, granted most funds come from private donors instead of imperial coffers.

Long war short, an army of 50 000 loyal to go over with the 143rd and according to legend, God himself came and blessed the army with his favour- giving them, and their descendants, powers. They continue to take over the open lands, join up with a group of tribes to kill the other inhabitants and create the country that is now Crius. The 143rd claimed this new land theirs and divided it into provinces among themselves and governed themselves, leaving the motherland with all their kin (elders and their kin).

I have a document where I kept all my lore and one beta, who'd previous read drafts, asked to read it and when going through the story of Crius she noted that although she didn't interrogate it when she read the first draft- it barely features by the end of point of what she's read but it is told- but the creation doesn't make sense because if they were these obsessive nationalists who want to conquer and expand for the sake of glory, they wouldn't have just split from the empire and would've continued ruling over it all even if the new land was still split between their families. They had the oppurtunity to come back with an entirely newfound confidence (getting powers from God) which they can spin as a right to stay in power and would've achieved their goal of glory by creating a new tier of class over a larger joint empire instead of the dramatic joult from everything they knew and starting something new.

And to be fair, it would be more characteristic to just have the expansion instead of changing their mind to a split. I've tried postponing the split (important it does eventually hanpper) but following this separation the source of the main storyline begins, which involves the 143rd as the heads of their new country that has a completely different structure of power than the old empire, so it doesn't work out smoothly. Or am I just overthinking this and using it as a distraction?