Is the future already predetermined? Were the Ramayana and Mahabharata written beforehand?

Saṅkṣepato mayoktāni sapta manvantarāṇi te।
Bhaviṣyāṇy atha vakṣyāmi viṣṇoḥ śaktyānvitāni ca॥

- Srimad Bhagavatam Mahapurana (8.13.7)

Here, Mahamuni Shukdev is shedding light on who will be Manu, Saptarishi, Devta, and Indra of the coming seven Manvantaras to Maharaja Parikshit.

That is, not only Ramayana, Mahabharata, or Bhavishya Purana, but the future of thousands of Yugas to come, who will be the kings, who will be Indra, who will be Prajapati, who will be the Sapta Rishi, who will be the Devta, etc., all these futures are predetermined.

When the time comes, this secret knowledge is imparted for the welfare of human society by the special people appointed by God with the permission of God.