The changes you have done are correct, all had to work with your changes. So in this case, I guess the issue with the attachments is related to the source forum(bbpress). The changes that you have done is the only thing that we could do from our side. And this kind of large work is out of our support. I'm sorry, but we cannot help you here.You should contact to professional forum migration services for custom migration, We only recommend gConverter or ProfProjects services for such projects. They have already done hundreds of forums to wpForo migration projects with the best result.
In case you want to say thank you !)
We'd really appreciate and be thankful if you leave a good review on plugin page. This is the best way to say thank you to this project and support team.