actually, in reality there would be
many increasingly severe problems with expanding an empire that big in pre-modern times. but most of these problems are hard to implement into a game in a way that isn't just incredibly annoying and/or boring.
imagine not having any map information in 'real time', but delayed by how long it takes an informant to travel from the location in question to your seat of power, and then having any orders you give delayed by that same time again before they reach the units/agents they were meant for - with the same delay again before you know if the orders even properly reached their targets, and then
yet again for any corrections you might need to take.
and that's not even taking into account that these orders wouldn't be automatically followed in detail, but would only reach someone in charge who might very well pursue their own agenda for power or wealth (or just plain survival) regardless of what would be best for the empire as a whole.
the same goes for tactical battles, to a degree, which means tactics are almost entirely set before the battle starts, with countless different potential scenarios taken into account in advance, and then executed by several dozens of individually thinking and acting officers. any changes that were made, and information about things that happen on the front, would need to be sent with messengers who may or may not make it to their destination.
and that's just
one issue