PowerShell: How to pass correctly parameter to new PS instance run by Start-Process

Question

Hello stackoverflow team, please could You assist on following question: How to pass correctly parameter to new PS instance run by Start-Process using arguments or ideally parameters.

I have found only one ugly way interpreting-concatenating string variable to script. This actually works but e.g. adds some spaces to string:

#ugly variable concatenation to script, but adds spaces
$whatToSay="Hello World"
$process = Start-Process powershell "-Command",'
    $greeting=\"',$whatToSay,'\"
    Write-Output \"I say: $greeting\"
    Read-Host
    exit ($greeting -eq \" Hello World \")
' -Wait -PassThru
Write-Output "Process exited with exit code: $($process.ExitCode)"

Result (note added spaces before and after "Hello World"):

I say:  Hello World 
Process exited with exit code: 1

Following two examples are based on powershell.exe /? (help):

PowerShell[.exe] [-PSConsoleFile <file> | -Version <version>]
...
[-Command { - | <script-block> [-args <arg-array>]
              | <string> [<CommandParameters>] } ]

Using Script-Block:

#does not work script-block argument
$whatToSay="Hello World"
$process = Start-Process powershell "-Command",{
    $greeting=$args[0]
    Write-Output \"I say: $greeting\"
    Read-Host
    exit ($greeting -eq 'Hello World')
},"-args","$whatToSay" -Wait -PassThru
Write-Output "Process exited with exit code: $($process.ExitCode)"

Using Script-String:

#does not work script-string parameter
$whatToSay="Hello World"
$process = Start-Process powershell "-Command",'
    param ($greeting)
    Write-Output \"I say: $greeting\"
    Read-Host
    exit($greeting -eq \"Hello World\")
',"-greeting","$whatToSay" -Wait -PassThru
Write-Output "Process exited with exit code: $($process.ExitCode)"

Both results:

I say: 
Process exited with exit code: 0

Thank You for tips.