Look the Tolkien elves are primarily Norse-Scandinavian but pull extremely heavily from early medieval vernacular Irish stories and especially the immram genre (ppl argue that this in turn could be derived from earlier sea journey epics like the Odyssey; that doesn’t matter). Assuming a basis in medieval texts, that the elves can sail to the Isle of Joy Valinor but everyone else would loop around to the east coast of middle earth should not be because to elves the world is flat. I would argue that this was a misunderstanding of Tolkien except I think Tolkien also believed and was troubled by this (elf flat earth). The thing is, the elves aren’t sailing on a flat earth because the ocean they’re on likely does not have its seabed on the earth. They’re doing something weirder, because they’re mimicking Irish and Old English stories about a universe in which the earth floats (earth usually given no shape, though a few ppl refer to it as a sphere) in a near-endless sea. It does have a seafloor but one totally disconnected from and much larger than the earth. They’re in a cosmic bath.
I’m on the train but iirc see: Bisagni, “Where Does the Sun Go At Night” for the Irish stuff. John Carey also but PSA he gets a bit speculative. If you know the voyage of st Brendan, there are some earlier stories too like that. Sailing to a heavenly island to the west is a standard Irish trope.
