I agree with your reasoning;
Felix is a bit unusual as far as being heard on an actual person, and I like the balance of a more usual middle name. I think
Felix goes well with something fairly solid and I prefer it with a longer name. I'd suggest perhaps:
Felix Theodore (my personal favorite)
Felix ChristopherFelix Frederick (if you don't mind them beginning with the same letter)
Felix AnthonyFelix SamuelFelix BenjaminFelix CharlesFelix Emmanuel (if you're okay with the heavy religious connotation)
Felix Jeremiah Felix Joseph