Yes you should start in the middle and work out, first on one side and then the other. I didnt start in the middle with my first quilt and I had a huge material bubble in the middle. It looked awful.

Thank you! This is complicated! LOL

Nice quilt. I usually cut my my batting about 3" bigger on all sides.

Thank you!

for straighter lines, go slow. don't push the quilt through under the foot just guide it. and every time you need to move your hands, stop, move your hands, then start again. it takes practice like everything else. but you'll get it!

Don't over complicate it. Keep it simple, and add complexity gradually as your confidence improves. Some find straight lines simple, I however can NOT make a straight line! I find meandering much easier! Another option is to follow your lines in intentionally wavy lines.

I have another suggestion for quilt number 2. Often I will put hearts or some other embroidery on my quilt tops. I do this before I put the backing on. I usually attach the batting and top using 505, rather than stabilizing. I embroider what I want embroidered by hand or machine. Sometimes I even minimally stitch in the ditch so that the batting is secure. Then, I add the backing and quilt all three together, sometimes adding more stitching in the ditch or some particular design once the back is at least minimally attached to the batting. This way, I don't have to pin the three together and I can embroider designs more easily. There is no reason you have to add your designs when quilting. They can be done beforehand. You can even embroider on the top w/o anything other than stabilizer.