[mill (Martin)]

Don't do this because you will end up with a very bad scope.
They stop grinding the mirror just before the parabolizing.
Figuring that last little bit cost too much time for small mirror manufacturers and that is why they use a barlow in the focusser (cheap and nasty).
You could still collimate the scope using the laser.
Use the barlowed method, there are many websites explaining the barlowed laser technique.
Since you already have a barlow in your focusser you don't have to use the barlow as they show you on those websites.