Populate a Name/Group column from a single line text column

Since you have a SharePoint list with ~20k items, it's likely that you're encountering performance issues when trying to update the list using Power Automate or other methods. Here are a few options you can try:

1. Use PowerShell: You can use PowerShell to update the email column for all the items in the list. You can use the PnP PowerShell module or the SharePoint Online Management Shell to connect to your SharePoint site and then use PowerShell commands to update the email column. For example:

```
# Connect to the SharePoint site
Connect-PnPOnline -Url https://contoso.sharepoint.com/sites/MySite

# Get the list
$list = Get-PnPList -Identity "My List"

# Get all items in the list
$items = Get-PnPListItem -List $list

# Loop through each item and update the email column
foreach ($item in $items) {
$item["Email"] = "email address removed for privacy reasons"
$item.Update()
}
```

2. Use Excel: If you're comfortable working with Excel, you can export the list data to Excel and then update the email column using Excel formulas or VBA macros. Once you've updated the email column, you can import the data back into the SharePoint list.

3. Use a third-party tool: There are several third-party tools available that can help you update large SharePoint lists. Some of these tools allow you to bulk update items, while others provide a user-friendly interface for updating list data.

Before making any updates to your SharePoint list, make sure to test the changes in a development or test environment to avoid any issues with production data. Additionally, you may want to consider breaking the updates into smaller batches to improve performance and reduce the risk of timeouts or errors.