If you are using a manual tamper, this video from @DavecUK would help.
I don’t have your machine and don’t know how it works. But, your shot temp seems to be on the higher side, unless it’s light roast. You might try losing a couple of degrees and see if it helps.
I have a Duo and my average grind size is 8-9 for medium roast.
Do you normally drink as an espresso ? If yes, you may consider diluting it to see if the “usually bitter and not balanced” tastes go away?
Perhaps, you can post a video of your shot prep. But, your shots look normal to me.
Blends and some light roasts can be difficult to dial in at times. As MRS said, the shot time can vary by coffee. I buy filter roasts and my average shot time is usually 21-22s on the lever.
If you prefer lighter roasts and fruity coffee, faster and longer shots (turbos), as Jake said, can be very nice. Recently, I have had a Kenyan from Zennor. It was lightly roasted, and I had to grind at 10.5 on the Duo compared to an average grind size of about 6. I had lovely cups at 18s for 1:2.5. Similarly, I had to grind at 9 for another coffee lasting just 15s for 1:2.5 to rid of a bitter finish.
Please note caramel, (dark) chocolates, floral, herbs, would have a bitter edge. They are good with milk.
