5e Cultivation Journey

In this world the cultivation journey is a big deal, but the majority of people don't have access to it. Even if someone has the ability to cultivate, they may not have access to the knowledge required to achieve it.   I want the level system to be in the hands of the players, they gain EXP how and when they want to. At the same time, I want to make the search for knowledge a bigger priority to the players than gaining EXP. With knowledge comes power.   To achieve this, the players gain EXP by absorbing monster cores, which can only be found in the brain of a monster. The size of the core depends on the monster's CR and the amount of EXP a core will give you depends on your own level in relation to that core's size (a level 5 character absorbing CR1 cores will get only a fraction of the XP from it compared to what a level 1 character would get). These monster cores are a valuable component for the creation of magical items, so there is a booming market for them.   On the other hand, players don't always know what they need to advance their cultivation. Classes, subclasses, spells and more, all are often withheld from the general populace or those who are not part of the trusted "inner circle" of an organization.