Got a 2.5G keg set up with some RO Water. It's carbonated over the past week or two but I've got a good 0.2 micron carbonation stone that I will be installing in some way in the future.
Adding a Stevia/Erythritol blend has yielded some good results but significantly reduced carbonation. I've been using cola candy flavoring from Flavour art for the time being and it does an alright job. I also add some citric acid to try to combat the bitter taste. Future additions would be anhydrous caffiene.
I think the solution is to definitely add the flavoring in situ and just dispensing a finished product instead of sodawater to a "syrup".
One thing I have to battle is the bitterness that comes from carbonation. I knew about this beforehand but I never realized it was so pronounced. I imagine adding the caffeine in will exacerbate this situation.