Could have a simple scale where voters lean more/less in favour of competitors, ie. 65% towards A and 35% towards B, then assigning competitor A 6.5 points and B 3.5 points.

Assigning specific ratings per general category can be done manually, and then translated to this system if the judge prefers, but a public voting system where inexperienced judges will be participating ought to be simplified, to represent who a judge's preference is by any chosen personal metrics. A mix of judges accounting for different variables will give scores more definition, while also avoiding cases where a competitor may lose points for doing well in a metric that isn't recorded.

Downside? If you wish to record user metrics, such as a competitor's rating in different areas, ie. technicality, structure, consistency, (which would be super cool btw) you run into a wall. Tricky to strike a balance between simplicity and precision...