I think the best identifier to use with a messaging app is a VOIP SMS telephone number.
The identifier that Signal uses is a cellular telephone number. I am very glad to tell you, however, that you don’t really have to use a traditional cellular telephone number. For example for my Signal service I don’t actually use a cellular number, I use instead a VOIP number that happens to have SMS service enabled. That VOIP number is not linked to any SIM card. My VOIP provider (VOIP.MS) uses two-factor authentication and, because it is not a cellular provider, is not going to fall prey to a SIM swap attempt. (See Being smart about SMS two-factor authentication.) This protective step costs me only 85¢ per month. I recommend it for all of your SMS two-factor authentication and I recommend it for your Signal service.