There’s a bulk import script for phpBB running on postgresql. Did you see it? discourse/phpbb_postgresql.rb at main · discourse/discourse · GitHub
However, I don’t think it has been used recently, so it’s probably broken and might not work with current phpBB versions because the Markdown storage format changed.
Whatever path you choose, I wouldn’t spend too much time on gold plating your solution unless you really want to. The current import scripts will be deprecated sometime this year…