Pulse cauliflower florets in a food processor until riced.
Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add cauliflower rice, cover, and steam, stirring occasionally, about 10 minutes.
Transfer cauliflower rice to a bowl and stir in cream cheese, rice vinegar, and sweetener. Place in the refrigerator to chill until cold, about 1 hour.
Mix the chopped tuna with Sriracha in a separate bowl to desired spiciness.
Spread 1/3 of the cauliflower mixture on a plastic wrap-covered bamboo rolling mat until you have a square roughly the size of a nori sheet.
Place one sheet of nori on top of the cauliflower rice square. Place 1/3 of the avocado slices and 1/3 of the tuna mixture along one edge on top of the nori. Carefully roll, starting from the edge with the fillings. Sprinkle with 1/3 of the sesame seeds and cut the roll into 6 slices with a sharp knife. Repeat with remaining cauliflower rice, nori, avocado slices, tuna mixture, and sesame seeds.