Yes, I think WisePilgrim has all or most these alternatives.
I WISH I had taken a picture of it. It is not so narrow, just extremely uneven. Like someone threw a bunch of stones together and then melded it all together. I think Amancio has been there, so I will contact him. I am bad with distance estimates, so I can’t give you a good sense of how high it was.
If you zoom in on the wikiloc tracks, you can clearly see it. It is the only spot on the tracks where you can see water on both sides.
https://es.wikiloc.com/rutas-senderismo/senda-costera-la-franca-pendueles-por-la-costa-10189855
It looks like you would have to miss all of the coastal part from La Franca to Buelna if you didn’t want to cross it. But maybe Wise Pilgrim or Amancio can help here. I have PM-ed them. Buen camino, Laurie
P.s, if you look at my pictures in post number 7, this is exactly the kind of rock that it is. I would say the bridge itself is at least 3’ wide, but very uneven and jagged.
EUREKA, if you put puente natural buelna into google images, you will see it. None that I see are taken FROM the bridge, just of it. But you can see people walking on top. The best one I saw was picture number six on this website.
https://mapio.net/pic/p-1043883/