Whatsapp already uses Signals encryption scheme. Also, Element (the Matrix company) is working with Meta on opening up Whatsapp, with encryption of course: https://matrix.org/blog/2024/09/whatsapp-dma/

It was not great for a long time but improved a lot in 2024

It improved a lot and has a new maintainer, your problems might be solved by now.

They need Google Messages and they need to enable it in the settings.

It uses matrix-rust-sdk (written by Element) and that uses the new vodozemac, so you’re safe

You go to networking jail for that.