I had the same problem recently. After failing to get my friends and wife to use Keybase (they tried it, but it just doesn't work on mobile beyond basic text chats with no image pasting).
Requirements were: secure, iOS/Android/Mac/Linux/Windows, multi-person chats, and multiple simultaneous/alternating device support (that means chat history needs to propagate to my phones and computers).
iMessage was not considered (because it supports only iOS & macOS) and WhatsApp was not considered (because Facebook).
After looking at Signal, Telegram, Keybase, and Wire, we ended up using Wire. It's not perfect by a long shot, but it is better than the others we tested and AFAICT better than any solution currently available.
But I had never heard of it until I really started looking around.
https://wire.com
Requirements were: secure, iOS/Android/Mac/Linux/Windows, multi-person chats, and multiple simultaneous/alternating device support (that means chat history needs to propagate to my phones and computers).
iMessage was not considered (because it supports only iOS & macOS) and WhatsApp was not considered (because Facebook).
After looking at Signal, Telegram, Keybase, and Wire, we ended up using Wire. It's not perfect by a long shot, but it is better than the others we tested and AFAICT better than any solution currently available.
But I had never heard of it until I really started looking around.
https://wire.com