New player looking for advice on keeping a large Empire together

This is my first run of CK3, not counting the Ireland tutorial. I started as a count in Brittany and have since gone through 3 heirs and 4 generations. I own England, Italy, the Holy Roman Empire, Francia, Carpathia, my own custom empire, and a bunch of other lands in Spain, North Africa, the Middle East, etc. Basically just a giant blob.

My last emperor was a Generous Diplomacy character and was able to keep this giant blob together very well. The Diplomacy tree is busted in my experience - he lived to 75 and was able to complete all 3 lifestyle trees, so he was gaining massive stats from his 20+ friends, alliances, and his Living Legend fame. His diplomacy was like 35. Most of his vassals had +100 approval. Whenever factions showed up he could just give gold for a free stress release. He literally looked like the Gigachad meme.

When he died, the crown fell to his only son, a kind of crappy Martial character. Everyone hates him and factions have immediately started popping up everywhere. Also Genghis Khan just spawned.

The only way I have found to deal with this is to essentially cheat - try to imprison a child duke, he revolts with about 20+ people, I siege him and RNG manipulate using saves so that he gets captured and loses the war, then execute everyone for dread.

That's a temporary solution and not how I'm actually supposed to do it, and I'd really prefer not to have to keep cheating my way out of revolts while Genghis is knocking at my door. What's my move here?