As such, I am curious to know how the custom image ecosystem of Vanilla OS is. Like, does something like this exist for Vanilla OS? (FWIW, there was another website that made browsing through these a better experience. But, sadly, I couldn’t find it.)

Well, there is Vib, and there are various custom images out there, but sadly i am not aware of any single website that list them. It’s all in all a much smaller project than uBlue and in things like that it shows.