I am using a singleton with dependency injection to get the default settings I want everywhere:

services.AddSingleton(new JsonSerializerOptions()
{
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    PropertyNameCaseInsensitive = true,
    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
});

And then it is available anywhere like so:

public class MyClass(JsonSerializerOptions jsonOpts)

Not sure if this is the best solution - hopefully I never have to change this, as that would warrant a potentially massive regression test.