I dont think I'd hesitate to jump that rheostat, if you want it dimmer you can just experiment with resistors to dim it a little but Ive never found it too bright at any setting other than full..

you could check if there is any resistance at full brightness. ( with a meter hooked to the pot and wires disconnected)

any resistance in series with the bulbs will tend to lower the brightness. I can;t remember how many bulbs are in the speedo , it could be there is more than one , like maybe 2? and one is burned out..

i remember the bulbs are not the same as the tiny ones used for dashboard illumination, but also small bulbs.