Bottom line
Choose Chatto on Appbox when you want the closest compact all-in-one option for private rooms, files, roles, voice, video, and screen sharing. Choose Rocket Chat for broader configurable communications, Mattermost when the real requirement is structured workplace chat, and TeamSpeak 3 only when replacing Discord voice is the main goal.
Keep Discord when its bot ecosystem, community discovery, moderation, Stage-style events, streaming, or familiar clients are essential.
Before switching
Pilot one representative community before migration. Test roles and permissions, moderation, threads, bots and webhooks, file history, notifications, mobile clients, voice quality, video, screen sharing, exports, and account recovery. Define backups and update ownership for every Appbox-hosted service. Discord data, bots, roles, and message history may require manual migration or may not transfer at all.