How to configure Serilog with different log levels for each namespace in my project? I want to do it via application.json config file

As you suggested in your configuration as well. It is possible to do so. Below is the sample configuration.

"serilog": {
"Using": [ "Serilog.Sinks.Console" ],
"MinimumLevel": {
  "Default": "Information",
  "Override": {
    "Microsoft": "Warning",
    "Microsoft.Hosting.Lifetime": "Information"
  }
},
"WriteTo": [
  {
    "Name": "Console",
    "Args": {
      "formatter": "Serilog.Formatting.Elasticsearch.ElasticsearchJsonFormatter,Serilog.Formatting.Elasticsearch"
    }
  }
]}

MinimumLevel config section dictates what should be default and overrides for specific namespaces.

1. Default log level is Information.
2. Classes in Microsoft namespace except Microsoft.Hosting.Lifetime should have minimum log level as Warning

As side note I registered my Serilog as

webBuilder.UseSerilog((ctx, lc) => lc.ReadFrom.Configuration(ctx.Configuration));

And formatter part is Trivial in context of the question but if you want to use it I refered Serilog.Sinks.ElasticSearch as extra package to use it.