I went 25 years with a j pouch and had no polyps in the rectal cuff, although a number were removed from the pouch itself and duodenum. At the time, I was on an annual scoping schedule. Then my GI biopsied a suspicious spot in the rectal cuff that turned out to be malignant. A CR surgeon removed it and fortunately took sufficient margin to eliminate risk of spreading. However, I believe my sphincter was compromised by that surgery. Five years later high grade dysplasia was found by the sphincter and my GI said that the sphincter was very weak. A second opinion at Mayo Clinic confirmed the diagnosis and an ileostomy was the only option they offered. I was very strongly against having the bag for quality of life issues it has and did further research on options that would avoid the bag. Fortunately, I found a surgeon who gave me a BCIR and I am very pleased with the results. Another similar procedure is the k pouch.
My thoughts are for you to continue frequent scoping, including several biopsies of rectal cuff tissue to determine if any dysplasia is present. With FAP, the short section of rectal cuff can contain colon tissue not removed by the original surgery, and this can become cancerous over time. Frequent monitoring should enable you to safely keep your j pouch.