For normal tempering below 400F (maybe 500F with added insulation) your oven will work as well as anything you can buy. I use a PID controlled toaster oven that controls temp ±1°F with no problem. How can you do better than that? A couple of comments on your build.
1. Be sure to use a good heat sink on the SSR, and keep heatsink in good airflow for cooling. That toaster oven is going to pull some current and create lots of heat on SSR. Did you order a 40 amp SSR? I've blown a couple of 25 amp SSRs due to excessive heat build up.
2. With the PID mounted where it's at, be sure to place plenty of insulation between hot oven and PID. If possible it would be good to have small fan mounted to blow air over PID for cooling. Perhaps extra ventilation holes in case for better airflow?
3. An old firebrick laying on bottom rack does a good job of adding mass to oven allowing a tad better heat control. I mounted the TC probe from rear of oven so tip of TC would extend into oven and be real close to center of blades in oven. Remember, only the tip of TC probe reads temperature.
About the only limitation of a PID controlled toaster oven I've found is size of blades that will fit.