For about three years I blamed the beans. Then I blamed the machine. It turns out it was the grinder all along, which is the most predictable plot twist in coffee and I walked straight into it anyway.
The blade grinder I'd been using doesn't grind, it smashes. It produces a pile of particles of wildly different sizes, and that variance is the enemy. The fine bits over-extract and go bitter, the coarse bits under-extract and go sour, and you taste both at once in the same cup. No amount of fiddling with dose or time fixes a grind that's inconsistent before the water ever touches it. A burr grinder crushes the beans to a roughly uniform size, and roughly uniform is the whole game.
I will not tell you how long I spent reading about stepped versus stepless adjustment, conical versus flat burrs, retention and grind speed, because the honest answer is embarrassing and the genuinely useful conclusion fit in a sentence: get the cheapest decent burr grinder you can and stop reading. I bought one. The next morning the coffee was better than it had ever been, from the same beans I'd been quietly resenting.
So: if your coffee is fine but never great, and you've been changing everything except the grinder, change the grinder. And then, like me, try very hard not to spend the following fortnight watching videos about water hardness.