Operation cancellation
Not sure if it counts as an answer, but the most important missing part was the following ASP.NET Core attribute:

public class AmbientContextAttribute : Attribute, IAsyncActionFilter
{
    public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next)
    {
        using (new Op(context.HttpContext.Request.GetDisplayUrl()))
        using (new Cancellation(context.HttpContext.RequestAborted))
            await next();
    }
}