I'd be fine with slowing down a bit later, or slowing down a bit, just not right now, and not by so much. Turn 2, with the Lizardmen in the Southlands and the Dwarves gone, I think we'll have the necessary breathing room to begin a massive build-up. Right now the Karaz Ankor is still a major, openly fighting enemy of the Skaven, and I think getting rid of them, and doing field tests on our weapons simultaneously, would be something to be done early.
Likewise, the Lizardmen would also be actively hunting down Skaven, and I don't want to risk the Southlands getting in contact with the rest of Lustria. They're noted to have a number of complete tablets with prophecies. Killing them off early I think would be rather important.
Those are(to me) the two main threats we'd be fighting early on. Take them both out, and we should have plenty of breathing room to do a full-scale upgrading to the Under-Empire.