Water made a bigger difference than my grinder upgrade
I put this off for ages because "water recipes" sounded like something only competition people care about. Then a friend brought over a bottle of the water they use and we brewed the same coffee side by side. The difference was obvious enough that my partner, who drinks coffee mostly to wake up, picked it out immediately.
Our tap water is hard. It's fine to drink, but it makes coffee taste muted and slightly chalky, and it scales the kettle in about a month.
What I do now is very low effort: I buy a low-mineral bottled water (check the label, total dissolved solids somewhere around 50–100 mg/L) and use it only for coffee. A 5 litre bottle lasts me roughly a week. I tried mixing my own from distilled water and mineral concentrates and it was good, but I'm not going to be weighing magnesium sulfate at 7am.
The grinder upgrade I did earlier that year cost about twenty times more and the improvement was smaller. Not what I wanted to learn, but here we are.