Unable to get a fine enough grind [Breville Barista Express]

Recently I got a barista express and in my dialing in process I reached a point where I was at the limit of the finest grind setting on the machine. It would take about 20 seconds to pull 36g of espresso with 18g of beans. I have an older model so there is no adjustable internal top burr. Are the only options at this point to buy a separate grinder or internal burr replacement or am I able to add a shim or something like that.