Simplest solution: in the Audio Project Settings, under Output, toggle the "Stereo" button until it becomes "Mono". Done.
From your 2-track output you will now get two identical tracks, without any stereo imaging. So ch1=full mono mix & ch2=full mono mix.
If you hook your audio outputs to a deck, this mono signal will be sent to the deck.
You could also remain in Stereo Out, but then you would need to center pan on all clips in the timeline. Select the tracks you need to "mono-ize", open the audio mixer, gang all tracks (the green tape icon), set the pan to center on one track (will be applied to all ganged tracks), then from the hamburger menu choose "set pan globally".
One caveat: many, but not all stereo tracks are fully mono compatible. The stereo mix would have to have been checked with a correlation meter, in order to determine possible phase cancellation issues when remixed to mono.
I'd do it on the timeline by panning tracks to the middle and remixing as needed. This way I can spit everything out without changing output settings, possibly bagging myself the next time I output. Ilike to keep that stuff as is all the time. My timelines are what vary.