I've been trying to figure out how it is tribes as small as 20-30 people have been surviving for thousands of years without having serious medical issues.

https://slappedham.com/10-most-isolated-and-dangerous-tribes-in-the-world/2/

I've figured it out now. In nature only the strong can survive, those who inherit weak genes tend to die off. Usually when they are still very young. Because of this only those with the strong genes survive.
This would explain why Tribes have so many children and how such small groups are able to survive thousands of years in isolation.