I don't have a theory about who RJ is, but I have a small theory about why it is NOT Bertram.

My personal opinion is that RJ WANTS PJ to suspect Bertram by throwing off obvious and fake clues:

1- In season 3 finale, the female accomplice of RJ first enters Bertram's room, why would she do that? why wouldn't she directly go to the intended room? It is to make PJ suspect Bertram

2- Let's say that RJ investigated all CBI people in order to better 'play' with PJ. And suppose that during his investigation he discovers that Bertram is a William Blake fan, nothing easier to him than to quote Blake, it would be the easiest and subtle way to direct all suspicions towards Bertram. If RJ was Bertram, I think he would be careful not to quote Blake

Did you grab any other clues? would love to know your opinion regarding this matter