there is no comparable option in the list - unless another platform with 300m+ adds micro-blogging option. The only nearest position could be if Instagram had a sidekick.
Having global decision makers (Journos, royalties, politico, celebrity) to join a new platform is NOT Easy.
discord is already the "deep layer" beneath the surface of twitter, so doesn't really make sense to say i'd "go there" -- although maybe through networks of semi-public servers you could kind-of recreate the energy
i'd probably go back to old school web forums tbh
Slack's threading is awful; also only niche-group scalable, no public engagement. So far mastodon feels very early-2000s internet: small friendly communities, actual reminders of the scale of the world, a lot of FOSS and artsy folks. Scooping up astro folks very quickly