Not sure the correct terminolgy, but can I resize a shape such that when it's placed in the centre of the original, the gap between the two outlines is equal?  Obviously easy with a square or circle but a rectangle scales such that the longer side reduces more than the narrow side for the same scale value.

Ideally I want to do this without having to scale x and y by different amounts.

Maybe there's a trace function that will create a second path that's exactly 5mm inside the original?

I'm going to keep playing but just thought I'd ask here too! Thanks.

Think I have it! Inset does what I want, once i get all my curves joined together properly. For some reason, despite snapping, the ends weren't quite joined...

https://logosbynick.com/offset-a-path-in-inkscape/