Students organise themselves into clubs and societies without the SU. We add massive value in the championing and supporting that we do but (and it’s a hard truth for me) we’re not essential when it comes to Opportunities actually happening.
I think the most compelling narrative that a students’ union has is that they’re aligned with you the student, not the institution. It’s in advice and campaigning that an SU plays a unique role that no other body is in a better position to do. So, I’d start there and build around it.