As a SEN parent, I find it so extremely frustrating that I am (now) more knowledgeable about SEN than the SENco, at every school we've attended.
I couldn't actually care less if the Senco doesn't have a teaching qualification (if non teaching role), but I do expect them to know about different disabilities, specific learning difficulties, autism, anxiety, and techniques that can help the children navigate life at school life generally, as well as in learning. They need to be calm and consistent in their approach, a great listener, and have knowledge about the trauma that families go through, to get any support.
They should be more specifically qualified than teachers, and help support the teachers spld knowledge gaps, rather than learning 'on the job' from other teachers who do not have enough spld/have dangerous or incorrect views on spld.
I have heard senco's spout utter rubbish about my child's issues, whilst simultaneously blocking us accessing the help they need with incorrect information (that I believed at the time).