Did it work in .net6?

I don't think that if you serialize this way the serializer will use our registered converters and options.

You should resolve our options with converters and use them:

var options = ServiceLocator.Current.GetInstance<SystemTextJsonSettingsOptions>();
var jsonSerializerOptions = options.SerializerOptions.JsonSerializerOptions;
//your custom settings
jsonSerializerOptions.WriteIndented = true;
//your custom settings
var jsonString = JsonSerializer.Serialize(content, options.SerializerOptions.JsonSerializerOptions);

But I'm not sure if that way all your properties will be properly serialized.

I would advice to use this little class:

[ServiceConfiguration]
public class SimpleIContentSerializer
{
    private readonly IContentStoreModelCreator _contentStoreModelCreator;
    private readonly IObjectSerializer _objectSerializer;

    public SimpleIContentSerializer(IContentStoreModelCreator contentStoreModelCreator, IObjectSerializer objectSerializer)
    {
        _contentStoreModelCreator = contentStoreModelCreator;
        _objectSerializer = objectSerializer;
    }

    public string Serialize(IContent content)
    {
        var queryParameters = new DefaultQueryParameters
        {
            TypeIdentifiers = Enumerable.Empty<string>(),
            AllParameters = new NameValueCollection()
        };

        var creator = _contentStoreModelCreator.CreateContentDataStoreModel<ContentDataStoreModel>(content, queryParameters);

        return _objectSerializer.Serialize(creator);
    }
}

I don't know what exactly did you need from your IContent content object but the code above will give you all the properties and some additional stuff.

Probably it would also work if you just used our IObjectSerializer, unless you have newtonsoft converters in your codebase.

Thanks for the reply. 

The snippet was working on .net 6 (with EPiServer.CMS.Core 12.21.7, EPiServer.ContentDeliveryApi.Cms 3.11.3).

I will get it a try using your suggestion.