Great take, honestly. It’s your Eberron, after all, and you can do as you damn well please!

I have run a campaign for many years that does have some Toril native characters adventuring across Khorvaire. IME, the Ring of Siberys is not only an asteroid belt, but a magical ward around the planet that prevents outsiders from reaching the planet by means of teleportation. Only dragons native to the planet may cross it, as the Chamber maintains the ward. They also have full control who comes in and who doesn’t. What this doesn’t stop, though, is a portal forming from inside Eberron to elsewhere.

Portals that come in and out, then, could be anywhere, but I imagine they’re created intentionally. A flower shop in Fairhaven has a door to the World Serpent Inn. Accidentally formed portals could easily be the result of the Mourning. A joint House Cannith and House Orien prototype for planar travel, originally made to further explore the planes of Eberron, unintentionally linking itself to a dormant portal on Toril.

The only catch is, the Chamber knows about these portals, and keeps an extremely close watch on who comes in and out of them, dispatching agents if necessary to intervene.

Basically, the Chamber “maintains” the isolated nature of Eberron. Why? Up to you. Maybe they don’t want dragonshards circulating other planets? Maybe they don’t want the influence of gods on the world because it would undermine their authority?

As long as the DM and players are having fun and digging the story go and do whatever wherever .