Hey, Zulip offers our Standard plan[1] for free for communities like yours. There is no restriction on message history. Plus the unique threading model of Zulip makes it easier to run communities[2]. If you folks are interested feel free to create an organisation in zulipchat.com and mail us at support@zulipchat.com for activating Zulip standard. We support exporting the data from Slack as well :)

[1] https://zulipchat.com/plans/

[2] https://zulipchat.com/for/working-groups-and-communities/