What about a large, circular animated mesh, some kilometers in diameter… it starts off flat, and you have the circumference deform/bend downwards or the middle bulge upwards so it eventually becomes a hemisphere…
I’d be tempted to fake the atmosphere by fading the skybox to black as you go up in altitude, and maybe have some haze material fade in on or above the landscape mesh.
I’ve been thinking about something similar for a spaceship launch sequence but haven’t implemented much yet. Let me know whats working and not working for you!