IMO playing cash will increase your BR faster than DON's....
Also, don't stop playing FR cash b/c you have a couple of bad sessions... If you jump around constantly you will not improve solidly in any one game. Eventually you'll play a session of DON's and bust out of 6/8 and get pissed and switch back to FR....just stick to one for a bit and learn it.
Don't do this operation just to build your roll ASAP.....do this to get better, learn and get to $500...

GL!