Maybe an answer is to have a control for each message that you can set to plain text or encrypted based on a cloud backed up key of encrypted based on a key only on this device. The you could message "hi mum, running late" without complications while being able to hard encrypt when you want?
Signal is already complication free (at least until your phone falls in a lake) making the control useless.
(And you probably don't need to worry about losing the 'running late' message in the lake... The need for good encryption and reliable backup on any given message is likely somewhat correlated.)
