Whether you apply straight to grad school or not depends on the goals you wish to achieve. Are you only eyeing Top 10 or Top 20 programmes? If so, depending on where past students have gotten into, it might make sense to go for a predoc/RA-ship first.
You also have a sufficient level of math so it shouldn't be a main concern for you.
Rather than taking more math, it makes sense to take PhD Micro and/or Metrics.