Scholars say that people evolve spiritually when they’re in Jannah. Does that mean the rewards people get in Jannah are not fixed? Or to rephrase, do the level of rewards in Jannah stay the same eternally?

For example, Mulla Sadra says:

As far as the Right-Side People are concerned, they are the people of the journey and actions. They are the pious people. They are of different grades according to their deeds. So they have grades of the rewards according to the grades of Paradises: For each of them grades according to their deeds. [Quran S:46 V:19]

Sadra, Mulla. Life after Death (Mulla Sadra) (p. 17). Kindle Edition.

Does this grade of reward stay the same eternally?

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Heaven is made in a way where who ever is in it will feel content with whatever level they have. So they would not feel the need to elevate to a higher level.

We have proof from some verses that the believers whose family also entered paradise but at a lower level, will be able to have their family raised to their level to be with them. This is an indicator that perhaps people can elevate in levels of paradise.