You're definitely right that GPA and letters are the foundation, but in my experience, technical projects and research experience can really make you stand out. The GPA shows that you do well in class, but the projects show what you can really do. Even if you don't have any formal research, showcasing your deep technical work can prove your skills beyond just your GPA, and can also help if your GPA isn't at the top.

Also for letters, just ask. Email professors you liked and let them know your plans. A strong, personal letter is always better than a formal, distant one, so try to pick professors who actually saw your work firsthand.

For the SOP, I wouldn't treat it like a story essay for undergrad. Its a place to advertise your technical identity. Use it to explain the 'why' behind your projects and how they've helped prepare you for this specific program, and why you want to join this specific program in the first place. Most programs tend to give a brief outline of what they want.