If you go into the relation editor in JOSM, there’s a button on the left to sort the members of the relation, which is what I used. It works as long as there are no gaps.

Guideposts (and other street furniture) are mapped in their physical position, not attached to the way they’re near. I saw you’d added the guideposts to the relation, so I looked it up on https://wiki.openstreetmap.org/wiki/Tag:route%3Dhiking and indeed they can be added to relations, with role=guidepost. I didn’t know that before, so I’ll try and remember to do it in future.

Previously I would have mapped guideposts as standalone and unrelated to anything (in OSM terms), but this approach does seem better. Ta :)