I expect that programming the AI on a slider might be a bit of challenge. Of course, I wouldn't mind additional options for AI difficulty.
In the meantime, I have two suggestions that helped me and that I pass on to you for what they are worth. I'm not a great player by any means, but can pretty much hold my own with hard AI most of the time and get killed by unfair. If I play FFA, I usually have some hard AI and some normal AI as the other players.
First, I keep track of where I rank among the players using the information in the lower panel of the screen. If I see that my ranking in terms of the size of my fleet is not in the top 1 or 2 in the game, I know I'll probably be in trouble if I don't concentrate on building ships. If you're not doing that already, its worth a try.
Second, you can create locked teams, with say, you and a normal AI, against a team of a hard AI and an easy AI. Or you and two hard AIs against an unfair and 2 easy AIs. By mixing up the teams, I can usually create matches that are challenging for my skill level, and still win (which is what I prefer!).