UsingTask must be moved outside a Target node.

Should these be instance fields? Also, an Action<string> looks like a better abstraction. Even better, you can use a TaskLoggingHelper here.

You can combine the three TextWriters with a single one like this:

    internal sealed class DelegatedTextWriter(Action<string> fWrite) : TextWriter
    {
        public override Encoding Encoding => Encoding.UTF8;

        public override void Write(string? value)
        {
            if (!string.IsNullOrEmpty(value))
            {
                fWrite(value)
            }
        }

        public override void WriteLine(string? value) => Write(value);
    }

And then create them in the task like Sgen.InfoWriter = new DelegatedTextWriter(Log.LogError);.

We should decide if the messages should have high importance, or which of them should.

We should make sure a .deps.json file is included in the package. You can see this guide for the best practices when creating MSBuild task packages.

We've had to abandon this approach because the Task was being run in msbuild running on .NET Framework when building in Visual Studio. We can't do that because we depend on implementation that lives in the .NET runtime to do some of the work. Would including a .deps.json file allow us to run the Task using .NET when building on Visual Studio?

No, that's not yet supported and is tracked in dotnet/msbuild#4834.

These are not required, now that Sgen is no longer a CLI tool.

Now that Sgen is no longer a CLI tool, you should add properties to the MSBuild task and to the Sgen class, and not construct command-line arguments.

These two are always true and we should update Sgen to remove all code that would run if they were false.

Not strictly needed, MSBuild binary logs keep track of when tasks start.