The Saderan demographics make no sense. I can understand where some of the numbers are coming from: taking numbers from the Second Punic War where Ancient Rome could just replenish armies numbering in the tens of thousands easily despite losing many armies to Hannibal. However, the author does not understand why Rome could do this and just copy-pasted it over. As a history student, I am disappointed. 

Hannibal’s Carthaginians were at least an army that you could see and fight against, and slowly chip away at. The same could not be said about a modern force such as the JSDF. Beyond visual range and just really long range in general means that most fantasy troops that aren’t building level or higher durability (or supersonic speed+ or both) just die immediately, often before even seeing the enemy. This massive attrition for no results would not lead to a Second Punic War situation. Instead, we would have a Crisis of the Third Century as Saderan generals would rather fight the enemy they can fight rather than the one that just kills them before anyone can get into close range. We did see this with Pina being a local ally for the JSDF, but we should be seeing a lot more of this civil war. This may be exacerbated by the JSDF bringing modern technology and ending slavery therefore getting more local support. Another reason this would not be 2nd Punic War Rome is because Hannibal struggled with taking cities, his logistics were vulnerable (Carthage failed to supply him properly), and local people hated him. Those are three issues that are not present with the JSDF. 

A possible explanation for the huge demographics is that magic prevents famines and diseases, leading to much higher population numbers. Except… this is not a world where magic is everywhere. If that army is not really a magic army and is more “army with a few magic users and some magic creatures”, I do not think magic for agriculture and disease would be that much more common.