I’ve been refining a do it yourself leash system for the last few years. After some fiddling, I found that fishing 4mm PolyUrethane cord (same material as most leashes) INSIDE dyneema amsteel rope works great. It has a stretch profile a lot like a typical leash, but “bottoms out” on nearly unbreakable and very durable dyneema cordstock.
The details about how to do the ends matter a lot. Maybe I should make a batch and sell them.
More info about this and other similar projects in the other thread about leashes