I think "Ashley Nathaniel" is a very handsome name.
As far as I know, "Ashley" is a last name and was first used as a male name. Nowadays is a unisex name. I'm a teacher and I believe so.
So, it you LOVE it, use it! I love the way you paired it with a strong middle name, as Nathaniel.
Ashley Nahaniel sounds great!
If in doubt about the name "Ashley", you could also go with the name "Ashton", which is a great name as well.
I think names with the beginning sound "ash" are Irish. So, you could also pair it with another Irish name. Perhaps like;
Ashley Rhys
Ashley Liam
Ashley Shane
Ashley Keagan
Ashley Kieran
Other ideas;
Ashley Maximiliano/Maximilian
Ashley Oliver
Ashley Leonardo
Ashley Jude
Ashley Mateo (mah-tey-oh)
Ashley Mathias
Ashley Elias
Ashley Donovan
Hope this helps and good luck in your search! :)
Hope this helps! :)