Signal is designed with the assumption that data is sensitive and you should err on the side of destroying it.

Facebook is designed more as a shared scrapbook, with the assumption that data is precious and you want to share it with your community, and you should err on the side of oversharing so you don't lose any precious moments. Signal is in no way a replacement for Facebook.

$50 per month for unlimited, not $50 per friend, so your solution only works if you only have 1 friend, so it would work for me (self-deprecating joke) but may not for GP.

Or, better, Mastodon or Matrix, which don't rely on a single, easy-to-target server.