Your request it’s a tough one haha. Keep in mind that Facebook also banned some users for using third party clients, it didn’t happens to every users, but sometimes happened. So, on android , a good idea would be isolate Facebook messenger in a secondary user. I don’t know if there is a web version of Facebook messenger for mobile. If there was, you could add the website to your homescreen and use it as application.
@ajz suggested beeper which seems a valid choice, though is a paid subscription service with a $10 monthly fee. However, you can also use basically every other messengers within only one application.