How can I redirect stdout and stderr without polluting PowerShell error output

Question

Problem:

I am trying to run a command via PowerShell and capture its stdout and stderr without printing them on screen (command is incredibly noisy and pollutes the console).

I want to capture the stdout and stderr in a variable and then throw an exception if particular strings are found.

My logic seems to be working and I can make the cmdlet fail/pass when I expect it to, however the output does not match what I expect, instead of returning the error message that I am specifying I get what I believe is the stderr from the command instead?

My code:

(Simplified for easier reading)
First cmdlet:

function Test-Validation
{
    [CmdletBinding()]
    param(
        [Parameter(Mandatory = $false, ValueFromPipelineByPropertyName = $true, Position = 0)]
        [Array]
        $ValidExitCodes = @(0),

        [Parameter(Mandatory = $false, ValueFromPipelineByPropertyName = $true, Position = 1)]
        [bool]
        $FailOnWarning = $true
    )
    $Validation = . pdk validate 2>&1

    if ($LASTEXITCODE -notin $ValidExitCodes)
    {
        throw "Module validation has failed. Exit code: $($LASTEXITCODE)."
    }
    $Warnings = $Validation -match 'pdk \(WARNING\)'
    if (($Warnings) -and ($FailOnWarning -eq $true))
    {
        throw "Module validation contains warnings.`n$Warnings"
    }
    Write-Verbose "Module successfully passed validation"
}

Which is called by a second cmdlet:

function Test-Module
{
    [CmdletBinding()]
    param
    ()

    try
    {
      Test-Validation
    }
    catch
    {
      throw "$($_.Exception.Message)"
    }
    try
    {
      Test-Unit
    }
    catch
    {
      throw "$($_.Exception.Message)"
    }
}

Expected output

PS /opt/script/test/> Test-Module
Exception: /opt/script/test/Test-Module.ps1:13:9
Line |
  13 |          throw "$($_.Exception.Message)"
     |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     | Module validation has failed. Exit code: 1.

Actual output

PS /opt/script/test/> Test-Module
Exception: /opt/script/test/Test-Module.ps1:13:9
Line |
  13 |          throw "$($_.Exception.Message)"
     |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     | pdk (INFO): Using Ruby 2.5.8 

As you can see it seems to be returning the output from the command I'm running (pdk) instead of the "Module validation has failed. Exit code: $($LASTEXITCODE)." message I am defining in the Test-Validation cmdlet.

Why am I not getting the error message I want and is there any way for me the achieve what I'm looking for? Alongside any code suggestions I would very much appreciate any further readings around my issues to so I can better understand these things.

N.B.: This is being run by PoSh Core on MacOS