Slice the chicken thinly on the diagonal into pieces about 2 inches long.Combine the sweetener, 5 spice powder, rice vinegar, avocado oil, sesame oil, soy sauce (or fish sauce or coconut aminos) and cayenne pepper (if using) in a medium bowl and stir.Taste for seasoning and adjust sweetness or saltiness to your preference BEFORE adding the chicken.Add the chicken pieces and stir well to coat.Marinate the chicken for at least 1 hour and up to 24 hours.Preheat the grill.Thread the chicken and red bell pepper pieces (or other veggies) onto the skewers.Grill for 2 -3 minutes per side or until the chicken is cooked through.Garnish with lime wedges and fresh cilantro if desired.