For secure communication via Messenger, i recommend matrix.org as the mainframe.
As the actual client for the chat, „Element“ is a very good choice!
Matrix defines a set of open APIs for decentralised communication, suitable for securely publishing, persisting and subscribing to data over a global open federation of servers with no single point of control. Uses include Instant Messaging (IM), Voice over IP (VoIP) signalling, Internet of Things (IoT) communication, and bridging together existing communication silos - providing the basis of a new open real-time communication ecosystem.
The functionality that Matrix provides includes:
- Creation and management of fully distributed chat rooms with no single points of control or failure
- Eventually-consistent cryptographically secure synchronisation of room state across a global open network of federated servers and services
- Sending and receiving extensible messages in a room with (optional) end-to-end encryption
- Extensible user management (inviting, joining, leaving, kicking, banning) mediated by a power-level based user privilege system.
- Extensible room state management (room naming, aliasing, topics, bans)
- Extensible user profile management (avatars, display names, etc)
- Managing user accounts (registration, login, logout)
- Use of 3rd Party IDs (3PIDs) such as email addresses, phone numbers, Facebook accounts to authenticate, identify and discover users on Matrix.
- Trusted federation of identity servers for:
1. Publishing user public keys for PKI
2. Mapping of 3PIDs to Matrix IDs
The end goal of Matrix is to be a ubiquitous messaging layer for synchronising arbitrary data between sets of people, devices and services - be that for instant messages, VoIP call setups, or any other objects that need to be reliably and persistently pushed from A to B in an interoperable and federated manner.
Btw:
This is a German forum. To get in touch with more users, it would make sense to write your posts in German.
A good online translator would be for example: https://www.deepl.com/translator