I have spent more money than I'd like to admit chasing better coffee, and almost all of it on the wrong end of the problem. Fancier brewers, expensive single-origin beans, a kettle with a temperature dial. The coffee got marginally better. The grinder is what actually fixed it.
For years I used a cheap whirling-blade grinder, the kind that chops rather than grinds. It produces a chaotic mix of dust and boulders, and that's the problem: the dust over-extracts and goes bitter while the boulders under-extract and go sour, in the same cup, every time. No amount of good beans rescues that. You're brewing six different grind sizes at once and tasting all of them.
A proper burr grinder grinds everything to roughly the same size, and suddenly all the other variables start behaving. The same beans I'd been writing off as "fine, I suppose" turned out to be genuinely good. I could finally taste a change in grind setting and know which direction to move next time, because there was a stable thing underneath to adjust.
That's the whole rabbit hole, really. You can read about water temperature and bloom times and brew ratios forever, but if your grind is inconsistent none of it lands, because the variable you're changing is swamped by the one you haven't fixed. Sort the grinder first. Then, if you must, fall down the rest of the hole. I did, obviously.