Cross-Platform Internal Communication App

I want to give my team a single, seamless place for me as a CEO to share latest news and updates about my company. We previously used Workplace app, but it is now no longer available, so I would like to build my own app. The app must run natively on iOS, Android, and in any modern web browser, sharing one real-time backend so conversations stay perfectly in sync.

Core features I cannot launch without are:
• Secure group messaging with searchable history.
• Fast, drag-and-drop file sharing that previews common formats.

Please also read the attachment with my complete wish list.

Push notifications, user presence, simple admin controls, and solid encryption should be built-in, but video calling can wait until a later phase.