First thing, maybe your existing stem is too low or is too long so the flat MTB bars are too far out front? Does it feel like you are stretched way out to the front while riding? A lot of the stock older MTB stems put the bars way out front and were too low, which doesn't make for comfy city commute riding.

Once the flat bars are set to a comfy reach and height, maybe add a set of curved bar ends to your straight MTB bars? I use those on my MTB and they give me several more hand positions. Plus, you don't have to change out anything on the existing bike, they are cheap and installation would only take a few minutes. Put handlebar tape over the bar ends for a little padding, and you're done. Try that for a while, and if that doesn't do the trick then think about different bars. Just keep in mind that different shaped bars might mean that a different stem length or height than what would be used flat bars.