How to improve the basis?

Hey guys! I've been working as a front-end developer since 2011, and "full-stack" for some years. I worked with react, vue, angular, node, ruby, and I know a lot about front-end and back-end, BUT, I always feel that something is missing. I feel like my knowledge is sparse, that I'm missing some basis. I'm a graduated computer engineer, but I think I need a better software engineer basis. Can anybody recommend any course? Don't know if I was clear enough..