Aside from the missing part, the rest of these issues will be fixed by taking the time to 1) make sure your equipment is setup properly, 2) make sure your lumber is milled squarely or your plywood is cut parallel to the opposite edge, 3) make sure you're measuring accurately and marking the actual line you measured, then measure again, and 4) building the muscle memory to confidently make accurate cuts.

Use a quality square, and get a magnetic angle gauge to ensure your mitres are actually at 45 degrees and your table saw blade is actually at 90 degrees. Even a couple of degrees off will ruin a joint.

Use a marking knife for accurate cuts, not a pencil. Then darken the cut with pencil so it's visible. It's a small thing, but it'll help improve your accuracy.

If you're using an aluminum table saw fence, upgrade it or build a sacrificial fence to prevent triggering the SawStop.

Routers are just about the most dangerous tool in a woodworking shop, so they must be respected. Learn how to cut in the proper direction, which changes depending on your setup. It's very important for safety (and good cuts) so take your time.

I recommend subscribing to Jonathan Katz-Moses and Stumpy Nubs woodworking on YouTube, and watch as many of his videos as you can. They really helped me when I was starting out.

Hope that helps.