Would it help if you told them you think Zuck is creepy? People often have a vague creeped out feeling about Facebook, Meta, and tracking. "I was messaging about (insert) here and started seeing ads for it on all my devices" might sell it and get them on your side.
Beyond that it's the pain point of them installing another app to communicate with likely only you, so leaning on things they already have might work. SMS/MMS may be a viable alternative, or maybe Delta Chat if they'd be willing to share an email address.
It's hard to deal with the saturation levels the big chat apps have so I wouldn't rule out having to keep WhatsApp, unfortunately.
Hundreds of people have already died in CECOT. I wouldn't be surprised if they put out a hit on him via back channels to gang members in the prison. Hell, they can legally do it now as an official act.