As Ive said- create a mono out in VST connection and then choose the tracks you want to export mono in the mixer.
Hold shift+al(its the quick link shortcut) to assign them a mono out all at once(in the routing section). Then you can go to batch export and export them all at once- tracks with mono output bus will render as mono, track with a stereo output bus will render as stereo.
Its a bit clumsy I know but there is no direct way o render mono traks to mono if they are assigned to a stereo output/Bus.
AS a tip- I always save to a new project marked stems before I do thid not to break the original project assignments.
J