Halex (halogen exchange) reactions typically don't work well, as exemplified by your 5% yield. I've seen Halex rxn being run at much higher temperature (180°C in DMSO), so you can give it a shot as you have the material in hand.
Wonder it you could de-methylate the methyl groups (BBr3), perform SnAR on the chlorines (NaOMe/MeOH, reflux) and brominate the pyridones (POBr3, reflux). Might be worth having a closer look at this option as well.
Thank you, I might try from the tetrachloro derivative, it may work. The SnAr (MeONa/MeOH) works very well at room temperature. In fact, the 2,3-dichloro-5,6-dimethoxypyrazine is obtained by this method described by Richard D. Chamber (1974 J Chem Soc Perkin Trans 25584).