What about Conversations?
I won't use Signal due to its reliance on Google Play Services, which is disabled on my phone. The Convesations app seems a better choice for many reasons: it also has double-ratchet encryption with a published spec (OMEMO), as well as OTR and stream management, and complies with open standards, works on self-hosted infrastructure, and does not need Google Play service, while still having very low power requirements. The main thing that prevents Conversations being close to an ideal chat application is that many of the larger providers of XMPP-based services (e.g. Facebook) refuse to support XMPP peering, but then lack of meaningful peering is a problem faced by all chat applications except email and SMS.