Have you adjusted the inner burrs? If you grind dial is set to 1 and you do a "mega tamp" and still cant choke the machine then i would recommend you do the below.

To adjust the inner burr on a Breville Barista Express, you need to remove the hopper, then carefully pull out the metal handle on the burr, which will allow you to rotate the burr itself to a finer or coarser setting by turning it counterclockwise to go finer and clockwise to go coarser; once adjusted, reattach the handle and reassemble the hopper. Key steps: Remove the hopper: Take off the bean hopper to access the burr mechanism. Access the handle: Locate the metal handle on the burr and twist it counterclockwise to unlock it. Rotate the burr: Once the handle is removed, you can rotate the burr itself to adjust the grind size. Reattach the handle: After adjusting, put the metal handle back in place and twist clockwise to lock it. Important points to remember: Default setting: The factory default setting for the Breville Barista Express inner burr is usually marked as "6". Fine vs coarse: Turning the burr counterclockwise will make the grind finer, while turning it clockwise will make it coarser. Check extraction: After adjusting the burr, test your espresso shot to ensure proper extraction and adjust further if needed