The solo mode seems too easy. I played it twice and won by more than double the AI's score each time. The issue I see is that the AI gets 1 brass per turn plus 1 for each of its constructed buildings, but doesn't start constructing buildings until it has 7 or more brass. So that gives the human player 7 turns where they have nothing to worry about, during which they can easily buy a few units and start collecting brass faster than the AI can. Or maybe I just got lucky during my two games and was able to buy units and draw them early.
I have a 50% win rate, so I don't find it too easy. Those 7 turns are designed to let you build your engine. You might just be a good player. Reduce those 7 turns if you want to make it harder.