You have done a fine job on your tshirt quilt!
I have found that I am the only one who knows where the weaknesses are in my quilts. Everyone else thinks they are perfect, so you be easier on yourself.
Improving something is always an option, but good work is good work and you need to give yourself credit for having done what looks to me to be a very good job indeed!
Next time <g>
your frustration may have come from the iron on batting, as you probably already know. A lightweight fusible interfacing would probably help you not have so many variables. And, for any considering using either product for the stabilizer on the tshirt fabric, always remember that interfacing, batting, has a give to it. By making sure that the "stretch" or "give" is always going the same direction, the quilt will hang differently.
Although, I still think your quilt is just great!