pete what I do is use chrome oil tan or real soft chrome tan as some of your chrome tan is stiff. I use 1/2" core rope, ( I think it is the perfect size for a handle ) the trick is to make sure you sew as tight to the core as possible. I have a CB4500 an use my right toe harness foot, when you sew don't sew straight, sew at an angle into the core as this will help pull the stitch closer to the core. I hope this makes sense to you.
The same with veg-tan but use 2/3 oz. Dye after you sew, dip in oil for 10 sec, wipe and let dry for a few days.
By the way you did a great job on this bag,. Keep up the good work.