why was it that he had to continually change the conditions for his barrier technique?
We saw Sukuna could land slashes on Gojo during that part of the fight, that alone should break the illusion that Gojo is invincible even though infinity is a bitch to deal with.
Admittedly, you could also acknowledge Sukuna didn't take Gojo seriously from the beginning and could have taken a part in this all. He earned his respect through the fight.
Give it some years. Gege obviously wanted to portray Sukuna as ultimately above Gojo, there's no further proof than Gojo's own words when the fight ended. That's why I refer to arguing against that as deepthroating, because both the author's intent and Gojo's opinion are against it.