I think your problem is two-fold. Upper tension may be an issue, but I think the biggest problem is your hand and machine speed.
Looking at the very first photo, the curves on the top are nice and flat and the stitches are of good length and uniform. Now compare the length of the stitches on the curves on the bottom. They are all larger, and not uniform in length.
I think your hand speed is too fast for your machine speed, especially on the curves.
Speed your machine up a bit, slow your hands down a bit. And remember, when you are on the curve you need to slow your hands down even more.
I would fiddle with speed first on practice sandwiches before I change the tension. And try EVERYTHING else first before you even think about changing the bottom tension, and even after that ... try them all again.