Short of running a phone with nothing else on it would it be possible to build a sandboxed WhatsApp instance that's bridged to a different client to keep FB off your actual device? This wouldn't solve chat privacy, but might help isolate the few group chats that might be unavoidable to leave.
You can also use Matterbridge in a similar manner: https://ibcomputing.com/setting-up-whatsapp-telegram-bridge-...