I think you're right, it's not possible without modifying the product so much that it can no longer be considered a true chocolate bar. I was going to post another comment in this thread about how I would make a chocolate bar. The sugar isn't even the hard part - the thing that makes sugar harmful is when it's 1) refined away from a whole-food source, 2) consumed in excess, and 3) part of a diet that is high in both sugar and fat, and low in other nutrients like fiber and phytonutrients. The first issue can be solved by using date sugar as the sweetener, which is just whole pulverized dates. Molasses (in small amounts) could also play a role.
The bigger problem is the cocoa butter itself. That stuff is really high in saturated fats, and chocolate cannot be considered chocolate without it's inclusion. If I were to prioritize health over adhering to a standard, I would use either some kind of nut butter as a replacement (ideally one high in omega 3, like walnut butter), or a healthier oil like canola oil. Hmm, maybe avocado could be a good choice.
At this point I think it starts to run into stability issues, so an emulsifier of some sort would be desirable. I wouldn't want to use industrial products like lecithins, so maybe something like chickpea flower. But then at this point we're gone so far from chocolate bar that we're probably talking about something that would be closer to a brownie texture.
Another thing I found in looking into this is that there are two main forms of cocoa powder/solids. The one used in chocolate bars uses a so-called "Dutch process" which leads to a product that is lower in flavanoid/polyphenol content. The other kind of cocoa, usually sold as "natural cocoa powder" has higher acidity, so a more sour taste. It requires either more sugar to taste good, is generally preferred more in baked goods, or in some cases will include an alkaline ingredient to neutralize the acidity.
So in pursuing health I end up removing literally every ingredient that makes a chocolate bar what it is. I guess I'm just finding out that I don't like chocolate bars after all. XD
It'd be interesting to experiment with these lines of thought and see what comes out...