You might have been better served leaving Cao Cao alone to serve as a buffer against Sun Jian. Cao Cao tends to go south ASAP and they usually clash around the Yangtze. Instead, rush Yuan Shao and Gongsun Zan, thereby nabbing some of the best land in the game. That is, if you want to try Liu Bei again.
Make sure to prioritize the corruption branch of the +income Industry building, as well as the -corruption Admin building.
I think your existing campaign is easily salvageable, though. You don't need to hold on to all that territory. Give some low-yield provinces away in peace deals, demobilize a few of those stacks (I count at least five in your screenshot) and spend twenty turns rebuilding your economy. Corruption at 77% is cataclysmic. Losing territory will help with that, some. Appoint admins to key commanderies (-30% corruption for each admin), research the -% corruption techs and get your house in order.
Getting conflicting reports from Steam forums saying Normal is cakewalk and that there are seldom any wars
The strategic AI is able to see when going to war is a Bad Idea (primarily, when you're in a coalition with powerful states) to a reasonable extent. Players who get declared on a lot tend to also not have too many such relationships. Also, the AI is far more likely to declare after you've had highly damaging battles, when your armies are battered. Or when you're poor. Though the above is mostly conjecture, it's been my experience over the last 50 or so hours of playing the game.
If you're still learning the ropes, I'd actually recommend Gongsun Zan for a reasonably challenging but straightforward experience. He has a great -corruption building and only ever fights on 1-2 fronts (with a corner start pos). Cao Cao and Liu Bei, who both start in/around the Central Plains, can be quite difficult if the diplomatic game goes against you. Liu Bei is also one of the most dependable coalition partners in the game, and when you play as him you lose access to that benefit.
For a more challenging but still satisfying start, I'd go with Ma Teng or Gong Du. In both cases, you start in a corner of the map that's easily defensible and has great strategic resources (silk, horses) in abundance. The greatest challenge is in the first ten turns, which, if successfully navigated, set you up for a great campaign. The fight against Lu Bu in Anding was a hell of a time. The aforementioned challenge revolves around the fact that Ma Teng and Gong Du start right next to each other, and both go for the same province capital on T1.