I also performed quite fine with imperative languages but… there was always something missing. I was keenly aware that I was riding on the shoulders of giants and had no real idea how stuff below worked (well, I did because I started with Assembly and C about 19y ago but I don’t know how the JS runtime works to this day). I wanted to know how the engine under the hood worked!
So I get the sentiment fully. 
To really bend your brain and learn a bit more about the mathematical side of programming – invest in FP. Doesn’t matter much if it’s Elixir, Erlang, LISP, Haskell, OCaml or what-have-you (although LISP is likely the closest to math and programming at the same time).
I recently bought Hands-On Data Structures and Algorithms with Rust but haven’t read it yet so can’t give you a recommendation for/against it.
The impostor’s syndrome is very real and you should to your best to fight against it. Academic prowess rarely wins the day; it’s the practical people who know technology good enough but are excellent communicators who get the big money in the end.