When explaining Signal to a non-tech-savvy person I’ll usually use one of two elevator pitches. Either something very similar to what you said:
“Signal has the modern features and convenience of iMessage without the artificial barriers imposed by Apple, everyone can use it and there is no vendor lock-in”
or
“Signal has the modern features and convenience of Whatsapp, but in contrast to Whatsapp it’s owned by a non-profit with a great reputation, not a near-universally disliked and distrusted big tech conglomerate (Meta/Facebook), and FWIW the founder of Whatsapp recommends Signal over Whatsapp.”
or just a simple
“it’s like iMessage or Whatsapp but, private and independent from big tech”
At the end of the day, I think it is important to frame our explanations or ‘pitches’ to the individual we are speaking to, you’ve got to find the aspect that appeals to that person, find the things they care about.