My understanding is 親友 (shinyuu) is the closest equivalent to English "best friend." In English, some people might say things like "My husband/wife is my best friend." Would using 親友 similarly be appropriate in Japanese? I'm not meaning to refer to them exclusively as this, I know there are other words for that, but would it make sense to also consider that person your 親友?
I assume you are asking whether calling your spouse a 親友 is completely unnatural in Japanese or not. The answer is that it is possible to say 夫/妻は親友です.
Yes, you may think about them as your 親友, but you will always refer to them as your romantic partner to other people.
If you refer to your romantic partner as your best friend to someone else, it might be understood that you don't see them as your romantic partner anymore and they have been "downgraded" to a best friend.
"My husband/wife is my best friend" using the word 親友 should be ok.