In this answer, I want to try to walk back the normative ideas we have about reward and punishment. Using Tanya as our source, we see that Tanya has come along to actually inform us that this system of reward and punishment is not the whole story. Just like everything in Torah, as we get older, we gradually but surely get deeper and closer to the more mature truth behind the sugya, and this is no different. The Lubavitcher Rebbe said that learning Chassidus is like being born again - even the impressions you got when you were a baby need to be "rewritten". Let's see if we can do this here.
The common conception of reward and punishment is that Hashem, being perfect and in need of nothing, decided in His goodness, to do us a favour and give us a chance to earn some reward (see Derech Hashem part 1). To establish this, He created good and evil, gave us a yeiter towards each, and rewards us whenever we overcome our yeitzer hara, and whenever we utilise our yeitzer hatov, and we get punished whenever we do the opposite, chas veshalom.
This idea is the very basic, immature version of a much deeper idea. If we neglect to graduate beyond it, we run the risk of losing people. In fact, many Jews who go off the derech, or who refuse to get on the derech in the first place, cite this idea as their main turn off. So basically stated, it sounds horrific to many people. We are created against our will, kicked out of paradise down to a rough world, and told to return by earning reward, and if we don't, we will be punished. During that time we have to encounter terrible suffering, and the odds are stacked against us. Hashem didn't even need to do this, so it seems all this is arbitrary, including all the terrible suffering. We need to go deeper!
So Tanya let's out the secret. The real, deeper reason that Hashem created the world is in order to establish a "dira betachton". Our goal is not to get back to heaven! Our goal is to bring Hashem down here, and make this world a comfortable, Godly place for Hashem (and each other and our own Godly neshamas). In fact, it goes even deeper than that. Hashem isn't into "places", He's not looking for a good mountain-top villa made out of beautiful sandstone with blue wave patterns. Hashem wants to dwell with us. A dira is a home, and a home is not a home unless you share it with your beloved, and Hashem's beloved is Yisrael. Bishvil Yisrael nivra olam (second Rashi on Bereshit).
So it's not about reward and punishment. It's not all about us. At least, not in that sense! It's all about us to Him, but from our point of view, it's all about Him (and each other). The worst thing that ever happened to Judaism was when we somehow absorbed this idea that everything is just a game to get into heaven and avoid hell, to gain reward and avoid punishment.
So the Tanya fills in more information to complete the picture. Firstly, Hashem gave us a very specific job to make this world comfortable for Him and our neshamas, and that is fixing the etz hada'at. Of course, He could have done that Himself, but He has confidence in us that we can do it and therefore really become His partners. Fixing the etz hada'at means we have to take every last detail of life and the human condition, and find a way to make it holy and have a proper place in a holy life.
Hashem's Tzibur is made up of Tzaddikim, Benonim and Rashaim (TziBuR). The Benonim and the Rashaim are the general population and the way they perform this mission is by having a yeitzer hara that makes evil pleasurable and appealing, and their job is to overcome this, either through avoidance and middot improvement, or teshuva (or both). Due to the lowly state a neshama has to be in for this to even be possible, these two groups are not capable of achieving very great heights of holiness and tzidkut, which takes tremendous sensitivity and distance from sin. As a result Hashem also created some special individuals who do not have a yeitzer hara and are not part of this. Their job is to be the shepherds of the rest of the tzibur. They spend their time learning, teaching, leading, and ensuring that the work the beinonim and rashaim is completed. Through prayer and tikkunim that they only know, they actually affect the elevations that the beinonim and rashaim begin when they do their work, and bring down the Godliness into the world in the place prepared for it by the beinonim and rashaim. They invite Hashem down into His world.
So the picture we have of reward and punishment is out the window now. This is not a game. This is Hashem's genuine attempt at making a real life of yichud with Yisrael, and He has put that in our hands. The real "reward and punishment" of this system is not necessarily about good and evil. That's just a special case, whereby some people are deliberately given a real connection to evil so that they can go and affect a tikkun, and work on it.
So the Tanya has listed out 3 different jobs we've been given, two of which involve connection to evil, one not. The schar, the wage, is in proportion to the effort put it. For beinonim and rashaim, the effort comes from overcoming evil, but for the tzaddikim, they have to put in effort in other ways. Both of these are equally precious to Hashem.
Perhaps the best way to illustrate this is by looking at human relationships. Starting with a father and his children, it's very clear that if one of the children really enjoys doing his father's will, the father gains tremendous pleasure from this child. The children who have to struggle gain a particular endearment to the father, as they struggle, even if this struggle means their accomplishments are much more meagre, objectively speaking.
As you wrote in your comment below, this good child gains pleasure from performing the will of the father, but I disagree that that means it is not an effort. The good child shows his devotion to his father by really dedicating himself to his father's will, and working very hard. The father would never say "this amazing house you built for me over 20 years of blood sweat and tears is meaningless to me because you enjoyed building it"! In fact, I'd go further and say that the fact that the son enjoyed doing it for his father adds to the fathers pleasure. Judaism does not say that pleasure is somehow a detraction from altruism!! A very important point! Altruism is the measure of where do you get your pleasure from, is it from your own desires, or from fulfilling the desires of others? The latter is not natural and is remarkable if it is achieved by anyone. The latter is the very definition of a Godly relationship of "I am for my beloved and my beloved is for me", which can be summarised as both parties in the relationship saying "I get no pleasure from serving myself, only from serving you". The converse is also true, "I get no pleasure from serving myself, but it does please me when you do things for me, because it's you". We all know that when we cook ourselves a meal, it's no where near as delicious as when a special someone else cooks us a meal, even if it's actually identical.
So to summarise. The Tanya completes a picture for us that makes the question fall away. Creation and life is not an arbitrary "game" for us to earn reward and avoid punishment. We are all going to Olam Haba, it's not about that. Life is a chance to build a close relationship with Hashem and each-other, and create a life for us, and create a home for us. The reward is the closeness, the goodness of a Godly relationship and home. Even deeper: the reward is each other, the reward is Hashem, may He be blessed! The punishment is the shame and sadness that comes from pushing this plan backwards instead of forwards. The reward is the pleasure we gain from serving others and seeing how it really does make a genuine difference to them (Avot 1:3), the punishment is seeing how we failed others, how we hurt them or worse... Hashem included. This goes beyond simplistic good and evil, and tzaddikim are key players in realising this plan. It is also notable that this is our only chance to achieve this. In Olam Haba it won't be possible...
As Ramchal says in Chapter 13 of Mesilat Yisharim, a chassid is someone who is motivated to serve out of love. They are not simply hoping to tick the boxes of yes-good, no-bad, but they sincerely wish to anticipate the desires of their master, and please them.
As the Lubavitcher Rebbe put it, when we accepted Torah, we did so b'mesirat nefesh. Included in that is our pleasure. So rewarding us by giving us pleasure in Olam Haba isn't going to work! The only thing that will work is schar mitzva mitzva: seeing how much it meant to Hashem is going to be the only reward we will accept, and the reward is great....