Mods are where it's at. The AI isn't competitive when you play fair (the vanilla game's idea of difficulty is just a multiplier to the crisis faction's strength). The only way for the AI to truly be challenging is for the AI to have an overwhelming resource advantage and mods do that best. I played with the Gigastructures mod and it was amazing. I was really skeptical of that mod before I tried it. I thought it'd just be power creep.

And it does have power creep. But it also introduces really well made, unique, and unbalanced challenges. Like, the blokkats are an enemy that is impossibly strong at first. First time around, they devoured half the galaxy before I could catch up to them. It was the first time in Stellaris where I was genuinely afraid I was gonna lose the game.

Similarly, other mods keep the game feeling fresh with more events and special planets and the likes.