Dynamic Parameters - with Dynamic ValidateSet
The problem is that you have all the dynamic logic inside scriptblock in the if statement, and it handles the parameter addition only if the path provided contains a semicolon (':'). You could change it to something like:
if($path.ToCharArray() -contains ':') { $FilePath = "\\$ComputerName\$($path -replace ':','$')"} else { $FilePath = $path}and continue your code from there
PS 6 can do a dynamic [ValidateSet] with a class: