Sounds like you might wanna consider a PhD in engineering. I had similar feelings when I applied to college: physics is my true love but I don’t see myself in academia. I’m in my senior year of my mechanical engineering degree now and applying to PhD programs so that I can work in the research and development side of engineering (which I would say seems pretty close to what you describe as “theoretical engineering”!) rather than have to work in a more mundane engineering position, while still working on things that have practical applications. You also definitely don’t need a PhD to work as an R&D engineer, but if you’re academically inclined it might be a cool experience.