How can I make 'rich chocolate' flavor?

Frankly, Hershey's cocoa powder is low quality. Buy a higher quality cocoa. One objective measure is the cocoa fat content.

This is from Harold McGee's Keys to Good Cooking, published by The Penguin Press, New York, 2010, p. 476:

Higher-fat cocoas make richer dishes. To compare the fat contents of different brands, check their nutrition labels.

For rich chocolate taste you need rich chocolate.

All cocoa powders intended to be mixed with milk to form a quick cocoa drink contain a lot of additives like sugar or milk powder in addition to the actual cocoa to make it a sweet drink and to keep it cheap. But all these additives take away the actual cocoa taste (which is actually intended because cocoa is bitter).

Instead you could use pure cocoa powder (pay attention to the quality) and add sugar to your own taste. But the richest chocolate taste can only be achieved by using real chocolate with a high cocoa content (at least 75%). Grate it very fine and mix the powder to your other ingredients.

A high quality drinking chokolate ist made the same way. Instead of mixing cocoa powder with milk, finely grated dark chocolate is used instead, which created a rich taste and creamy texture.

Unless you are adding a substantial amount of fat (like using half & half or heavy cream), I would recommend using dutch-processed cocoa powder instead of regular cocoa powder.

Regular cocoa powder is highly acidic and generally requires lots of fat and lots of sugar to make it palatable. Dutch-processed cocoa powder has been treated with an alkali to neutralize the acidity and tame the sharpness quite a bit. It usually won't require near the amount of fat to taste good. If you look at most protein powders or shake mixes that are chocolate-flavored, you'll likely see that they include dutch-processed cocoa rather than regular cocoa powder (it'll usually say something like "cocoa processed with alkali").

Maltodextrin is primarily used as a bulking agent for "high intensity" sweeteners like Sucralose and/or acesulfame potassium. It may have a slight sweetness but it wouldn't alter the taste in a perceptible way. Xanthan gum is usually used in tiny amounts to help thicken the mixture and should have little effect on overall taste.

I don't think your title question is answerable, because I'm not sure what you mean by "rich", however, I'll tell you what I know about the question at the end of your post. Isomaltulose is a natural disaccharide. Google suggests it tastes similar to sucrose with half the sweetness. So, it probably adds sweetness to the powder. Maltodextrine is a starch based polysaccharide, it can range from slightly sweet to flavorless. Some have a perceptable flavor, but I doubt you to know it is there if a flavor like chocolate was present. It is usually added to powders as a bulking agent. Xanthan gum is also a polysaccharide, most often used as a thickening agent. Xanthan probably adds no flavor. So, other than adding some sweetness, these ingredients are probably serving other functions like, reducing clumping, bulking, and thickening (xanthan) and keeping the mix in suspension when liquid is added.

You can try mechanically "powdering" (grating or finding some other way to turn into powdered form) real chocolate (not compound chocolate) and mixing it in, if you really need it in powdered form.

You can also use compound chocolate if you want to use something more affordable, depending on whether or not compound chocolate tastes richer to you or not.

On using cocoa/cacao powder vs actual chocolate:

Real chocolate is made by combining roasted cacao/cocoa beans with other ingredients (normally sugar) and grinding them together to make a moldable paste (the real chocolate we know). If you mix 70 parts cacao based substances, plus 30 parts sugar (and other substances), then you get 70% chocolate. Or if you just grind 100% cacao, then you get 100% chocolate.

On the other hand cocoa powder is made by taking the cacao bean, and extracting its vegetable fat from it (called cocoa butter). The pulp left behind (normally in powdered form) is basically cocoa powder, which can be used for cooking and baking. Dutch processed cocoa powder on the other hand takes the pulp and processes it further (through the dutch process). Either way, cacao powder is basically cacao with its fat content removed.

On real chocolate vs compound chocolate:

While real chocolate is made of ground cacao beans, compound chocolate is made by combining cacao powder with some form of vegetable oil, palm oil, etc. Compound chocolate is what you normally see used by more affordable "chocolate" candies.

One of the main benefits of using compound chocolate however is that it doesn't melt as easily as real chocolate. Real chocolate can start getting real soft and melt on temperatures above 27 C. If you have powdered real chocolate in your mix, and it gets heated above that temperature, then you can just imagine the chocolate melting and clumping with the powder together.

On what kind of chocolate to use

If you're fine with the chocolate manufacturer's ingredients (which includes sugar and other chemicals) then you can look for n% chocolate bars (80%, 70% etc). If you just want 100% chocolate, then try to look for 100% chocolate bars, then just add some powdered sugar if desired (or other sweetener), to your mix.

As to chocolate taste, cacao beans from different geographical regions and different estates taste differently from each other (like wine). The way the cacao is fermented and roasted also affects the taste. In my case, the cacao I use from 2 different places produces 2 fairly distinct tasting chocolate bars.

In your case, I suggest looking for a manufacturer that produces the "right" chocolate taste for your purposes.