The Silmarillion takes place over thousands of years with lots of characters who come and go. The main purpose of the frame narrative should be a familiar point of departure for the viewer into ancient Beleriand. Therefore, the frame narrative should be relatively stable, with a familiar cast of characters from episode to episode. I would recommend against a separate frame narrative for each season, lest the story of the frame narrative distract from the actual story of the film. Thus, I suggest the frame narrative be set in one location where characters can tell all the stories to each other. I can think of two settings in the Lord of the Rings satisfying these criteria: Rivendell before the Fellowship departs, and Minas Tirith after the downfall of Sauron. In both locations, the characters can be relaxed storytellers. In Minas Tirith, with the War of the Ring mostly resolved, there is less tension, and the entire story may be told. In Rivendell, however, the outcome is still in doubt, and the tension of the mission to destroy the ring can drive the frame narrative.
I suggest that the frame narrative be set in Rivendell as the fellowship prepares to depart, mainly as stories told to Frodo and the hobbits. The storytellers could be Elrond, Glorfindel, Gandalf, Aragorn, Arwen, Gimli, Gloin, Legolas, Boromir, Bilbo, or any of the other elves present.