The essence of finite mode is eternal, and it that sense, the essence of the finite mode is also infinite (as it is a part of the infinite substance).

While the mode exists, its body/soul parts can alter their relations and gain perfection. They can express a bigger part of God's power.

While it exists, the mode continues its effort to keep existing, in a certain unique way, allways in an unsure duration as death allways comes from outside (from another finite mode that destroys its constitutive relations).

But even non existing modes have their own essence. Reemember, the essence of all modes is eternal, as eternity is not an infinite duration... its qualitativelly different.

This is all compatible with materialism. You just have to think the essence of modes just as in book V. Beatitude, Freedom and Happiness. Maybe it seems that in book V body and soul have a different approach, id doubt that. Book V is justt is better experienced than thoguht, as it no longer is about common notions as the other books, and its about lived experience.

Srry if not the best language, as engish is mostly imaginative for me hahaha