The average length of the human rectum is about 10-12 cm. The point of a j-pouch is to replace the rectum if it is removed. Most of us have had our rectum removed due to IBD or cancer (or related disease). If you are lucky enough to not require removal of your rectum, there would be no need of the j-pouch.
What you would be having is an ileorectal anastomosis, and is what is appropriate in your situation. There is no reason to undergo the additional surgery and face the significant increased risks.
Yes, 6-8 bowel movements per day will be an adjustment after living with chronic constipation, but it should be a welcome relief. There should not be an issue with continence. Also, understand that initially, you will have a higher frequency, but things will slow in the first 3-12 months.
Jan