For you information, "Set-PnPUser" works only if your account is already granted as site collection administrator.

Import-Module SharePointPnPPowerShellOnline

$tenantName = "<YOUR_TENANT_NAME>"
$localeId = 2057 # UK
$timeZoneId = 2 # London


#Connect to SPO Tenant
$cnx = Connect-PnPOnline "https://$tenantName-admin.sharepoint.com" -UseWebLogin -ReturnConnection


#Get Tenant CSOM instance
$tenant = new-object Microsoft.Online.SharePoint.TenantAdministration.Tenant([Microsoft.SharePoint.Client.ClientRuntimeContext]$cnx.Context)
function Remove-PnPTenantSiteCollectionAdministrator{
    param([string]$siteUrl, [string[]]$users)
    $ctx = $tenant.Context;
    $targetSite = $tenant.GetSiteByUrl($siteUrl);
    $ctx.Load($targetSite);
    $ctx.ExecuteQuery();

    $users |%{
        $user = $targetSite.RootWeb.EnsureUser($_);
        $ctx.Load($user)
        $ctx.ExecuteQuery();
        if($user.IsSiteAdmin){
            $user.IsSiteAdmin = $false;
            $user.Update();
        }
    }
    $ctx.ExecuteQuery();
}
function Get-PnPTenantSiteAuthor{
    param([string]$siteUrl)
    $ctx = $tenant.Context;
    $targetSite = $tenant.GetSiteByUrl($siteUrl);
    $author = $targetSite.RootWeb.Author
    $ctx.Load($author);
    $ctx.ExecuteQuery()
    [pscustomobject]@{
        LoginName = $author.LoginName
        DisplayName = $author.Title
        UserPrincipalName = $author.UserPrincipalName
        Email = $author.Email
    }
}

#Get current UserName
$tenantWeb = Get-PnPWeb
$tenantWeb.Context.Load($tenantWeb.CurrentUser)
$tenantWeb.Context.ExecuteQuery()
$currentUser = $tenantWeb.CurrentUser



#Get All sites then execute
Get-PnPTenantSite | %{
    $site = $_;
    
    #Grant current user as admin if it's not already it's own
    if($site.OwnerEmail -ne $currentUser.Email){
        $site.Url | Set-PnPTenantSite -Owners $currentUser.Email |out-null
    }

    $user = Get-PnPTenantSiteAuthor $site.Url

    try{
        $ctx = $tenant.Context;

        $targetSite = $tenant.GetSiteByUrl($site.Url);
        $web = $targetSite.RootWeb;

        $timeZones = $web.RegionalSettings.TimeZones
        $ctx.Load($timeZones)
        $ctx.ExecuteQuery()

        $timeZone = $timeZones | ?{ $_.Id -eq $timeZoneId }

        $web.RegionalSettings.LocaleId = $localeId
        $web.RegionalSettings.TimeZone = $timeZone

        $web.Update()
        $ctx.ExecuteQuery()
    
    }
    finally{
    
        #Remove Current User as admin except for it's own
        if($site.OwnerEmail -ne $currentUser.Email){
            Remove-PnPTenantSiteCollectionAdministrator $site.Url $currentUser.LoginName |out-null
        }
        #ensure owner is the real owner
        if([string]::IsNullOrEmpty($site.Owner)){
            $site.Url | Set-PnPTenantSite -Owners $res.UserPrincipalName |out-null
        }

    }

}

For you information, "Set-SPOUser" works only if your account is already granted as site collection administrator and is not compatible with PnP contexts.

Import-Module SharePointPnPPowerShellOnline

$tenantName = "<YOUR_TENANT_NAME>"
$localeId = 2057 # UK
$timeZoneId = 2 # London


#Connect to SPO Tenant
$cnx = Connect-PnPOnline "https://$tenantName-admin.sharepoint.com" -UseWebLogin -ReturnConnection


#Get Tenant CSOM instance
$tenant = new-object Microsoft.Online.SharePoint.TenantAdministration.Tenant([Microsoft.SharePoint.Client.ClientRuntimeContext]$cnx.Context)
function Remove-PnPTenantSiteCollectionAdministrator{
    param([string]$siteUrl, [string[]]$users)
    $ctx = $tenant.Context;
    $targetSite = $tenant.GetSiteByUrl($siteUrl);
    $ctx.Load($targetSite);
    $ctx.ExecuteQuery();

    $users |%{
        $user = $targetSite.RootWeb.EnsureUser($_);
        $ctx.Load($user)
        $ctx.ExecuteQuery();
        if($user.IsSiteAdmin){
            $user.IsSiteAdmin = $false;
            $user.Update();
        }
    }
    $ctx.ExecuteQuery();
}
function Get-PnPTenantSiteAuthor{
    param([string]$siteUrl)
    $ctx = $tenant.Context;
    $targetSite = $tenant.GetSiteByUrl($siteUrl);
    $author = $targetSite.RootWeb.Author
    $ctx.Load($author);
    $ctx.ExecuteQuery()
    [pscustomobject]@{
        LoginName = $author.LoginName
        DisplayName = $author.Title
        UserPrincipalName = $author.UserPrincipalName
        Email = $author.Email
    }
}

#Get current UserName
$tenantWeb = Get-PnPWeb
$tenantWeb.Context.Load($tenantWeb.CurrentUser)
$tenantWeb.Context.ExecuteQuery()
$currentUser = $tenantWeb.CurrentUser



#Get All sites then execute
Get-PnPTenantSite | %{
    $site = $_;
    
    #Grant current user as admin if it's not already it's own
    if($site.OwnerEmail -ne $currentUser.Email){
        $site.Url | Set-PnPTenantSite -Owners $currentUser.Email |out-null
    }

    $user = Get-PnPTenantSiteAuthor $site.Url

    try{
        $ctx = $tenant.Context;

        $targetSite = $tenant.GetSiteByUrl($site.Url);
        $web = $targetSite.RootWeb;

        $timeZones = $web.RegionalSettings.TimeZones
        $ctx.Load($timeZones)
        $ctx.ExecuteQuery()

        $timeZone = $timeZones | ?{ $_.Id -eq $timeZoneId }

        $web.RegionalSettings.LocaleId = $localeId
        $web.RegionalSettings.TimeZone = $timeZone

        $web.Update()
        $ctx.ExecuteQuery()
    
    }
    finally{
    
        #Remove Current User as admin except for it's own
        if($site.OwnerEmail -ne $currentUser.Email){
            Remove-PnPTenantSiteCollectionAdministrator $site.Url $currentUser.LoginName |out-null
        }
        #ensure owner is the real owner
        if([string]::IsNullOrEmpty($site.Owner)){
            $site.Url | Set-PnPTenantSite -Owners $user.UserPrincipalName |out-null
        }

    }

}