I think your theorie has some edge. The only argument i would put
against it, is that he starred in only two (i think) episodes. Which is
not a lot. But for the rest i think he fits the glove.

1. he is
NOT PJ. I think that theory is really bullshit. You cant hold on to a
"fight club" like scenario for so many seasons. Ridiculous theory in my
opinion.

2. All female suspects. It's defintely a he.

3.
anyone within the cbi-team. (and then i mean the close colleagues. They
are far too caught up in catching criminals and much to serious about
it, AND just not smart enough.

So if RJ is a lot like PJ then we need to look at the characteristics of PJ:

he is: charming (especially to women), manipulative, unconventional, nihilistic to a certain extend and playfully competitive,

Besides
that he needs to have a lot of money to organize all these tricks he
played on Jane and i think he should be good looking enough to
accompany his manipulative charms. (Which kinda deletes Laroche and
Minelli for me)

Furthermore he has a fucking big ego (he took
vengeance on Janes portrayal on TV) but i am sure he would charm his ego
away unless he would be pushed to the very limit.

I believe
that no one in real life could really be Red John, and have so much
power over so many people (the only one would come near would be a sect
leader like Styles, which is a little too obvious suspect)

But
if I'd need to pick one on character then i'd also go for Washburn. He
had the same playfullness as Jane, likes to manipulate, likes to
outsmart the other, likes to take risks because of the boredom of
everyday existence (which makes him suitable for a nihilistic  American
Psycho 's Bateman-like character, and has enough time and wealth to keep
Jane busy.

So i go for Washburn too.