multiple Nimh vs dry cell
Just let it be dim. The average voltage of an alkaline battery is something like 1.3 Volts during discharge. The range is 1.55 down to maybe 0.9V

The average Voltage of a NiMH battery during discharge is around 1.2V. From like 1.35 down to 0.9.

These are just typical numbers. Some devices may cut off at higher than 0.9V or lower. But the main point is that batteries don't have a single voltage. They have a range as they progress from fully charged to discharged. Comparing NiMH to Alkaline, the top of the range is a bit different. The average is less different. And the end of the range, on the discharge side, is about the same.

So just use your NiMH batteries unless the dimness really bothers you.