Hangouts Chat will be available for consumers before OG Hangouts is EOL. I haven’t messed with it a great deal, but if it’s anything like Slack it’ll be great. Very feature-rich. That said, it will not have end-to-end encryption.
Whatsapp does have E2E encryption, but Facebook owns it and keeps it in the same shared app storage on iOS, so Facebook could theoretically snoop. Given recent revelations, it’s become clear that FB really is not a reputable ethical organization, not even up to the standard that we expect from billion-dollar companies. So I would not use it.
If you and your friends are looking to move to a new platform, the obvious answer is Signal. It is fully E2E encrypted, open-source, takes both security and privacy seriously, and is run by a non-profit foundation so it doesn’t have perverse monetization incentives.
Alternatively, if everybody is on iOS just use iMessage. It’s secure too, and frictionless.