For example, on WhatsApp, use the whole 25 character limit for profile name. Examples:
Bob Moved To Signal.org
Alice MovedTo Signal.org
CharlieMovedTo Signal.org
Say Signal.org, not Signal, because they won’t know it’s an app.
Use your about section too. Same on Discord, Steam, Instagram, etc.
Neat idea, or, you could just tell them directly and be like “hey I’m not gonna respond here due to [reasons], come find me on signal if ya wanna chat”
People are usually more engaged when you communicate to them directly
Both. You got time to respond to everyone, go ahead but I suggest an auto reply bot.