Contacts: export all contacts, copy over, import (that is the most clean way, you can attempt to sync via BT but my advice is do what I wrote)
Messages: there was a script around for going from N9->Jolla1 might be here or on together.jolla.com, but in the end, maybe it is time for a fresh start

Forums: will have a look the next days, was thinking about it a while ago

As you are new to SFOS, best starting advice, everything installed from openrepos may brick your device during a softwareupgrade, if you use OR make sure to read release-notes carefully, unapply all patches and best do not upgrade for a couple of days after a release if you got no time to recover it in the afternoon. Must-haves are the same as with N9, just some of them are missing, still.