DrForinor So 1:3 with this coffee, at 1.5 notches coarser than the less pleasant 1:2 shot, worked well.
Do you feel that you need to adjust anything, given the shot was as you described?
I wouldn’t adjust anything with this coffee, until a few shots in succession show the same fault.
If the next coffee/successive brews with this coffee start becoming sharp/tart, try a slightly finer grind. If you can’t shake the sharpness/tartness with grind adjustment, or the shot is very sharp/tart go longer on the ratio.
If shots with another coffee are flat/silty/muddled in flavour without sharpness/tartness, shorten the ratio, if this doesn’t fix it, grind coarser. If shots taste balanced at long ratios, but are just too weak & lacking intensity, try a shorter ratio and grind finer if tartness/sharpness creep in.
Bitterness can be inherent in coffee, some coffees just have more than others, without knowing the coffee itself it’s hard to advise. So look for other defects as well as the bitterness (chalky/powdery mouthfeel, lack of acidity). Over-extraction is a very specific bitterness, very drying and smoky, dominating the flavour. It is often very difficult to do in espresso at 1:2 or 1:3 even if you tried hard. I don’t think advice to adjust based on bitterness (in general) alone is very useful.
I find sharpness/tartness a better indicator (as opposed to pleasing, fruit like acidity bearing in mind some fruit are sour - it’s that puckering sourness we want to avoid) as this is pretty much always under-extraction. Also if the shot is so intense that it is preventing you from determining quite how it tastes, go longer.