We can help you for sure, we have migrated forums with more than 13 million posts and lot of attachments as well.
It's also very important to keep in mind redirects and verify SEO on page after the migration because traffic can decline after then but things are done in the right way it won't...
The Image Optimizer addon of @truonglv is causing issues with missing attachments, maybe it's related:
https://xenforo.com/community/threads/image-optimizer-for-xf-2-0-paid.140261/post-1663730
Hi Peter,
Who is currently paying the server? You'll need access the server in order to switch the hosting. If you are able to get access to it then you can hire Todo10 and we'll be happy to make the switch to any other server.