There are no fully open/auditable android phones. All of them have privileged binary blobs. An end to end chat service where there are no options permitting full accountability of the client software and operating system is largely security theater.
Even if you do all that, it is not an official option, let alone a recommended one. The recommendation is to accept the google or apple terms of service.
Moxie even went as far as to say he would actively do anything in his power to discourage or stop the use of third party clients.
