genuine guestion – how much difference to the taste does the grinding really make? Is it just to do with grain size?
yes, and a good grinder will have consistent grain size, it enables you to grind coffee that matches your machine perfectly so you get the correct dose and ideal pour time. this will mean better tasting coffee.
it’s all trial and error though you slightly overfill the basket tamp gently and firmly (assuming you have bought a nice shiny stainless steel tamper) if it pisses through too quickly the coffee will be thin and bitter, i then adjust the grinder until it sounds like the machine is going to choke but then the pour starts after a few seconds and lasts about 20sec before blonding*. or if the machine chokes you adjust the grind/dose/tamping the other way.
a bit of a faff but once done i only ever adjust the grinder half a turn if a new batch of coffee requires a slightly different grind.
*when the coffee goes pale as nothing is being extracted from the grounds