My vote’s for DIY Mattermost – I’ll take the control anyday.
For a hosted solution, I’ve not noticed mention of Gitter. (Did I miss it?)
Its free unlimited history, full slate of integrations, threads support, and now being part of Matrix seem attractive options.
No, nothing’s perfect.
And, full disclosure – I personally abhor Slack, and generally pass on the gamer-centric Discord as well.
And you probably want to “pick your corporate overlord” …