I agree and always thought so.
I said this in that Saint Shanks/Garling thread but I always thought the Gorosei were humans first and foremost, because they all belong to human Celestial Dragon families. And ofc like you said there was that one line about Imu being one of the twenty kings which pretty much deals the deal on him/her.
I think a lot of OP fans not having reading comprehension strongly applies here. When Oda has the Gorosei reference humans as if they themselves are not human, Oda is trying to make the reader realize that the Gorosei are hypocrites. Because they think they aren’t human, but in fact they are human. They may be immortal but they are still human. It is part of what makes them so despicable.
And this is how some irl rich people think. Irl rich people start to believe they are special or that they are a transcendent form of human because they don’t experience the same struggles everyone else does. Because being rich disconnects them from the human experience and deludes them into believing they are special. This is why Oda wrote the Celestial Dragons this way, because irl rich people are actually this evil.
Anyway yeah, Gorosei and Imu were obviously human initially.