here's how it plays in my head (and how it played on Black Panther's Quest too, btw).
10k years ago: Wakanda is founded around the vibranium mound, providing a technological jumpstart for the nation and creating early friction with its neighbors.
After struggles against other African nations as well as Lemurians and any other weirdness from the magical realm and maybe the odd space alien, Wakanda closes its doors on the world FOREVER. The policy of staying inside the borders becomes a religious and fanatical cultural attribute as does murdering any outsiders who attempt to come across.
Over the next 9k+ years Wakandans DO sometimes leave the kingdom but for only two reasons.
1) once in a while a dangerous object in the larger world comes to light (something like the Evil Eye or the Serpent's Crown) and the ruler of the time sends out a small strike force to find the object, secure it and bring it home to place in wakanda's vaults. The Outsiders are just too primitive to be allowed to have access to such powers.
2) Yemandi, the Leonardo of this already advanced culture, does a walkabout to "test her new technology" and to also secure dangerous items that the primitives outside wakanda shouldn't have.
She runs afoul of various great powers in antiquity, including Morgan LeFay and the Deviants. Eventually she returns to Wakanda and stays.
That's it. Aside from these, NO ONE leaves Wakanda, AT ALL, ever, until T'Chanda (you guys will say Azzuri but Chanda predates Azzuri and I added the
T' to keep things cleaner) meets Captain America during WW2.
Cap convinces him to provide aid against the Axis powers but, even then, Wakandans do NOT participate in the activities of the outside world. It is only after the USA explodes nukes in Japan that T'Chanda says, "Okay, the children are juggling hand grenades in a pool of gasoline. We need to step in."
Observers are dispatched (like 20, max) to set up shop around the world, keep an eye on these fools and, if they get too close to wiping everything out, step in.
T'Chaka, being privy to his dad's POV decides it's time to break with tradition a little. Wakanda needs a real diplomatic presence in the modern world so he opens the doors SOMEWHAT. Very limited access to Wakanda for specific outsiders with still nearly zero interest or interaction with outsiders from Wakandans themselves.
T'Chaka's trying to slow-walk Wakanda's integration into the modern world (from both sides) to give everyone time to catch up. It seems to be working but, when he's assassinated, everything goes crazy. The Wakandan Council wants to close the doors again- to hell with the barbarians. if they kill themselves off, Wakanda will still be safe. T'Challa, who has grown up at his father's side, traveling with him from time to time and seeing how the outside world is goes the other way. Rather than closing the doors, he decides he needs to get a CLOSE, PERSONAL look at these fools. he contacts the Fantastic Four and ultimately joins the Avengers to spy on everything himself.
After some years with them he deems the outside world Mostly Harmless and continues his father's open door policy. he's still slow walking the integration but moving faster than T'Chaka was. Also, he, like Yemandi, feels a bit tied down by his duties as king and keeps finding reasons to go on adventures. This causes frictions at home.
This is the foundation I'd base my BP on. Some version of this. It's basically how we set things up on the show.
This is just the BACKSTORY, not the plot of what I'd actually do.