Forwarding tab completion to another script

I'm trying to use Register-ArgumentCompleter to implement subcommand completion with the following behaviour:

> mycommand <tab>
subcommandA subcommandB subcommandC

> mycommand subcommandA <tab>
optionA1 optionA2

> mycommand subcommandB <tab>
optionB1 optionB2

I would also like to split each subcommand into its own script in a commands/ directory and discover them dynamically. I have achieved call forwarding and top-level completion with the following code:

Function mycommand {
    [CmdletBinding()]
    Param(
        [parameter(Mandatory = $true)]
        [String] $command,
        [parameter(ValueFromRemainingArguments = $true)]
        [String[]]$arguments
    )
    
    # Forward all arguments
    & "$PSScriptRoot\commands\$command.ps1" @arguments
}

Register-ArgumentCompleter -CommandName mycommand -ParameterName command -ScriptBlock {
    param($commandName, $parameterName, $wordToComplete, $commandAst, $fakeBoundParameters)

    return @(Get-ChildItem "$PSScriptRoot\commands" | %{ $_.Basename } | Sort -Unique)
}
    
Register-ArgumentCompleter -CommandName nrm -ParameterName arguments -ScriptBlock {
    param($commandName, $parameterName, $wordToComplete, $commandAst, $fakeBoundParameters)
    
    # (1) I land here with the correct `$fakeBoundParameters.command`. Now what?
}

I have tried to call back into TabExpansion2 from (1) with the following code:

    # Remove mycommand and subcommandX, replace with subcommandX's script path
    $command = $fakeBoundParameters.command
    $subtokens = $($commandAst.CommandElements | Select-Object -Skip 2)
    $inputScript = "`"$PSScriptRoot\commands\$command.ps1`" $subtokens "
    
    $(TabExpansion2 -inputScript $inputScript -cursorColumn $inputScript.Length -options $null).CompletionMatches

... but CompletionMatches comes back empty. I'm also unsure about passing the cursor position, to which I don't have access from my ArgumentCompleter.

Is there a simple way to forward the completion request to the subcommand's script?