I believe there has not been many opportunities for battle preparation since we're not charging into battle. I'm sure if something came up then our qm would give us a stopping point, and the hypothetically vote would be what Jacob does to prepare for the event.
Also, Attempting to build a small timeline:
This is back in October when we first get the book. It's the first time we get Lobo and the farm meeting.
The first 'raids,' as our eldest brother, the heir and Lugal, leads a counter force against this force. We also see their army is
very untrained, but this is near the beginning of recorded human history so they get a pass.
This is the beginning and return of 'our,' brother Isaac's ill fated reconnaissance mission. Massive Mystery Army of a Multitude of Species is Meet. Preparations begin.
The attack comes.
What I find curious is the use of horns for communication. This is clearly before magic lore or research exists, but the use of horns feels too abstract to come from nowhere. It also means the massive army somehow keeps itself feed for long enough to hit us in the same season. Granted, prior sections gives the impression they were already marching from the interior of Europe or Eastern Europe, so any attack is coincidental to a greater conquest.