Example V9+ Umbraco CMS Serilog Configuration to write to additional File location

appsettings.development.json

{
  "$schema" : "./appsettings-schema.json",
  "Serilog": {
    "MinimumLevel": {
      "Default": "Information",
      "Override": {
        "Examine.Lucene.Providers.LuceneIndex": "Debug",
        "Examine.BaseIndexProvider": "Debug",
        "Examine.Lucene.LoggingReplicationClient": "Debug",
        "Examine.Lucene.ExamineReplicator": "Debug"
      }
    },
    "WriteTo": [
      {
        "Name": "File",
        "Args": {
          "path": "warren-logs/log-.json",
          "formatter": "Serilog.Formatting.Compact.CompactJsonFormatter, Serilog.Formatting.Compact",
          "rollingInterval": "Day",
          "buffered": true,
          "retainedFileCountLimit":  "2"
        }
      },
      {
        "Name": "Async",
        "Args": {
          "configure": [
            {
              "Name": "Console"
            }
          ]
        }
      }
    ]
  },
  "Umbraco": {
    "CMS": {
      "Examine": {
        "LuceneDirectoryFactory": "TempFileSystemDirectoryFactory"
      },
      "Global": {
        "Smtp": {
          //"From": "your@email.here",
          //"Host": "localhost",
          // "Port": "25"
        }
      },
      "Hosting": {
        "Debug": true
      },
      "RuntimeMinification": {
        "useInMemoryCache": true,
        "cacheBuster": "Timestamp"
      }
    }
  }
}

Taking this one step further and making it possible to view the logs in the backoffice log viewer:

Step 1:
Configure a custom File-zink to write to and configure Umbraco to only write Fatal-errors to the "old" log path.

"Serilog": {
    "MinimumLevel": {
      "Default": "Information",
      "Override": {
        "Microsoft": "Warning",
        "Microsoft.Hosting.Lifetime": "Information",
        "System": "Warning",
        "SixLabors.ImageSharp.Web.Middleware.ImageSharpMiddleware": "Warning"
      }
    },
    "WriteTo": [
      {
        "Name": "UmbracoFile",
        "Args": {
          "RestrictedToMinimumLevel" :  "Fatal"
        }
      },
      {
        "Name": "File",
        "Args": {
          "path": "../Resources/Logs/log-.json",
          "formatter": "Serilog.Formatting.Compact.CompactJsonFormatter, Serilog.Formatting.Compact",
          "rollingInterval": "Day",
          "retainedFileCountLimit": "30",
          "buffered": false,
          "shared": false
        }
      }
    ]

NOTE: Added buffered:false and shared:false.

Step 2

Implement your own Umbraco.Cms.Core.Logging.ILoggingConfiguration and return the path to where you're storing the files.

Switch the it in the DI during startup after Umbraco has registered it's dependencies.

services.AddUmbraco(_env, _config)
    .AddBackOffice()
    .AddWebsite()
    .AddComposers()
    .AddIntranetAuthentication()
    .Build();

services.Replace(new ServiceDescriptor(typeof(ILoggingConfiguration), typeof(MyCustomLoggingConfiguration), ServiceLifetime.Singleton));