Balancing (Re)building Foundations and Research
This feeling is pretty normal at that stage. (I also guess that the feeling that one should know foundations better does never really vanish for many people.) To keep up your pace, but still broaden your foundations I suggest that you


choose some books you'd like to go through and reserve some time every week to work on it,
embrace the fact, that you will be able to pick up new stuff in your field more quickly than during graduate studies,
be aware that you'll pick up more and more foundations "on the fly" while doing research.