I only started by website 2 weeks ago, so I have only done a cursory review. Since I am using WordPress, I will probably start with BBPress even though my first impressions are that it feels outdated and lacking features.
Ultimately, I would like something simple for new tech users, that my users are likely to use elsewhere or (perhaps dreaming) would create their own forum in, so Google Groups or Communities would be an obvious choice since anyone with an Android phone has a Google account, but Google Groups feels so outdated and Communities isn't really a forum. The Indonesian government offers a Moodle site for schools, so I wonder if I could use that but haven't tried it (and my first impression is that it's quite complicated for K-12 teachers who are new to technology.) (Or a LMS that could be installed on Rachel??)
Ideally, users could have a single sign-on for my WordPress site, MediaWiki and forum. It appears that the venerable phpBB could do that.
I would love to know which ones work best on old browsers & phones and are relatively light in data use. The obvious answer to that is probably using an old system such as phpBB but I don't know for sure.
I also would like to be able to zip the forum to a searchable archive and provide it on the Rachel, so that offline users have the same access to information as the online ones, but that's not critical.