For normal end user average usage signal is the best option available, specially for family since they may already be used to the flow and UX of it. Simple and straight forward. All the "bad" things you read are about nerds being annoying and not liking a very particular specific thing and thinking that specific thing should be the only focus.

So just make people use signal. It's the best and simplest way with the most common features for individuals and small groups. A simple download, in a common known place on a store without confusing people with differences between a protocol and a client and with and onboarding experience most are already familiar and ok using.

Even so you still need to make sure that the app does not have battery optimizations turned on, but that applies to all apps used for communication that are not blessed in specific phones (like facebook and whatsapp already having that setting by default because vendors make it so).