I think you need to give Kyoshi's book a read as there is a lot of light shed in Avatar Kuruk's hidden stories which make him a very good arrgument for strongest avatar. Also i feel when it comes to comparing avatars the avatar state and how many avatars they have access to shouldn't be part of the argument and each avatar should be based off of their own skillset to avoid the more recent avatars very easily and unfairly taking the crown
One critical point you failed to mention is that Korra LOST access to her past lives when Rava was destroyed BEFORE she learned metal bending or mastered spirit bending. My interpretation is that after Season 2, Korra's Avatar state is just Korra+Rava with no past life abilities. Similar to Wan entering the Avatar state. With this in mind, I think Kyoshi's case for being the strongest is a lot... well stronger.