From a 700MB json file, how do I list all the keys in Powershell?

I tried

$obj = [System.IO.File]::ReadLines((Convert-Path -LiteralPath names.json)) | ConvertFrom-Json
 
$keys = @() 
 
foreach ($key in $obj.GetEnumerator()) { 
  $keys += $key.Key 
} 
 
Write-Output $keys

But after over 24 hours it had not completed.

I need the key names so I can

1. Delete irrelevant info and make it smaller
2. Convert it to csv (the key names are required, otherwise PS just uses the first object and ignores keys which are not present in the first object)

The JSON is a version of this one (though 200 megs smaller): https://kaikki.org/dictionary/All%20languages%20combined/by-pos-name/kaikki_dot_org-dictionary-all-by-pos-name.json