Problems upgrading to Laravel 8: "Conclusion: install guzzlehttp/guzzle 6.5.x-dev"

I've been struggling for a few days while I upgrade my Spark project to Laravel 8.

I am getting this error:

$ composer update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for laravel/spark-aurelius *@dev -> satisfiable by laravel/spark-aurelius[9.0.x-dev, dev-master].
    - Can only install one of: guzzlehttp/guzzle[7.0.1, 6.5.x-dev].
    - Can only install one of: guzzlehttp/guzzle[7.0.x-dev, 6.5.x-dev].
    - Can only install one of: guzzlehttp/guzzle[7.1.x-dev, 6.5.x-dev].
    - Conclusion: install guzzlehttp/guzzle 6.5.x-dev
    - Installation request for guzzlehttp/guzzle ^7.0.1 -> satisfiable by guzzlehttp/guzzle[7.0.1, 7.0.x-dev, 7.1.x-dev].

Taylor has mentioned that the Laravel installer also must be upgraded, so I have done that. I removed the global Laravel folder (/c/users/seth/appdata/roaming/composer/vendor/laravel on Windows), then adjusted my Environment Variables to point to /c/users/seth/appdata/roaming/composer/vendor/laravel/installer/bin.

$ laravel --version
Laravel Installer 4.0.4

This didn't fix the composer error in my project, unfortunately. So now I was thinking that I have to update the Spark installer?

I cloned a new Spark repo from Github ( https://github.com/laravel/spark-installer ) and updated my Environment Variables to point to the new Spark. But I'm not sure it has been updated:

$ spark --version
Laravel Spark Installer 3.2.1

More importantly, when I did a composer update in the Spark installer, I saw this line:

  - Installing guzzlehttp/guzzle (6.5.5): Downloading (100%)

Could this be the issue, the Spark installer is not using the new guzzlehttp/guzzle 7.0.1?

I tried updating the composer.json value for guzzlehttp/guzzle in this Spark installer repo to:

"guzzlehttp/guzzle": "^7.0.1"

But this gives me a lot of errors when doing spark new test-project-laravel8:

$ spark new laravel8-test-v10
'laravel' is not recognized as an internal or external command,
operable program or batch file.
Downloading Spark...
PHP Warning:  file_put_contents(C:\dev\trash/laravel8-test-v10/spark-archive.zip): failed to open stream: No such file or directory in C:\dev\spark-installer-v4\src\Installation\DownloadSpark.php on line 59

Warning: file_put_contents(C:\dev\trash/laravel8-test-v10/spark-archive.zip): failed to open stream: No such file or directory in C:\dev\spark-installer-v4\src\Installation\DownloadSpark.php on line 59
PHP Warning:  ZipArchive::extractTo(): Invalid or uninitialized Zip object in C:\dev\spark-installer-v4\src\Installation\DownloadSpark.php on line 105

Warning: ZipArchive::extractTo(): Invalid or uninitialized Zip object in C:\dev\spark-installer-v4\src\Installation\DownloadSpark.php on line 105
PHP Warning:  ZipArchive::close(): Invalid or uninitialized Zip object in C:\dev\spark-installer-v4\src\Installation\DownloadSpark.php on line 107

Warning: ZipArchive::close(): Invalid or uninitialized Zip object in C:\dev\spark-installer-v4\src\Installation\DownloadSpark.php on line 107
PHP Notice:  Undefined offset: 0 in C:\dev\spark-installer-v4\src\Installation\DownloadSpark.php on line 120

Notice: Undefined offset: 0 in C:\dev\spark-installer-v4\src\Installation\DownloadSpark.php on line 120
PHP Warning:  file_get_contents(C:\dev\trash/laravel8-test-v10/composer.json): failed to open stream: No such file or directory in C:\dev\spark-installer-v4\src\Installation\UpdateComposerFile.php on line 52

Warning: file_get_contents(C:\dev\trash/laravel8-test-v10/composer.json): failed to open stream: No such file or directory in C:\dev\spark-installer-v4\src\Installation\UpdateComposerFile.php on line 52
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - The requested package laravel/spark-aurelius could not be found in any version, there may be a typo in the package name.

... etc.

Any thoughts on what I am doing wrong?