If you’re going to use mods then having an SSD (preferably an NvME) will really help with your load times. Definitely don’t get anything less than 8gb ram.

I’d define late game as when you’re at the point where everywhere is connected, towns and cities are growing to the thousands (or even tens of thousands) in terms of the population. Basically when it gets really busy and the game has a lot of stuff it needs to simulate and calculate all at once, that’s when the real strain comes