I don’t see anything wrong with any of what you are doing. As we said before, nothing wrong with your shot either.
Is that a levelling tamper? Does it do 30 - 40 lbs pressure? I am not sure loosening the spring tension to apply a little lower pressure would help.
DW about the shot time. Focus on getting the taste right. Are they dark, medium, light roasts?
If you are drinking >= medium dark, you may get a bitter taste if the roast development cycle isn’t optimised. Dave can explain it better. If the roast profile is light / medium light, a less developed roast can give woody, mushroomy finish.
Stale beans could be another reason. Assuming the beans are not stale and not darker roasts, it’s potentially a brew malfunction. Are there any fines in the cup?
Based on the bitterness you explain, loosen your grind by 1 or 2 full marks on the Duo, see if that helps. Place a filter paper at the bottom and see if that helps.
