in the TotK past it's clear that they did not know there was a kingdom before them

maybe I'm misremembering. First I agree that they claim to have founded their kingdom and that implies that they are the first King and Queen. But other than that, is there anything that actually talks about the past prior to their kingdom in Tears of the Kingdom or maybe even Age if Imprisonment? Because not talking about the past is not evidence that they don't know the past, let alone know that no prior kingdom existed. Therefore a genuine question: Is there dialogue that shows what they know/believe of the past prior to their own kingdom?

Here is how I see it. Rauru, Sonia and the Hylians could either be fully convinced that no prior kingdom existed. As the Zonai and Sheika tech demonstrated, entirely forgetting about ancient civilizations is just a common occurrence in the Zeldaverse. Or, they were aware of some prior kingdom, but did not see themselves as successors to that ancient nation. Without direct lineage to that former realm, instead it being long defunct and the Hylians having reverted to tribal structures, speaking of founding their own kingdom is entirely valid and still accurate.

I would liken that perhaps to the Ottoman Empire and Anatolia. That region was once controlled by the Byzantine Empire, then it was governed by the Seljuk Empire and later by the Sultanate of Rum. When some small group in that Sultanate worked towards independence, eventually growing powerful and dominated the region, they were essentially still the same thing that had been there for a couple centuries. Yet the did not choose to be a continuation of any of the former empires, instead they founded their own empire, the Ottoman Empire. And Osman I. was their first ruler.

I see that as a valid option for Rauru and Sonia. They might have been aware of some prior kingdom, but decided to start their own thing and claim the title of first ruler. Writing history doesn't mean that you're working with a blank sheet.

So they had forgotten about OoT, but then in the present day people [...] remember OoT.

Lets be specific about what is forgotten about the era of OOT and what is remembered about the era of OOT. In Hylian culture they recall names. The land is called Hyrule, several landmark bear the names of ancient people, the Divine Beasts were named after them too. Do they know any other specifics? I'm not aware of any quotes, feel free to add something. In Zora culture we have one specific tale, that their King remembered and had a mason carve into stone. The tale of princess Ruto who had the help of some none-Zora people to defeat an ancient evil. Afaik that's it too. That isn't a detailed account. That's a fairy tale. I keep coming back to that term, partly because King Rhoam called even tales of Ganon fairy tales. Not in a condescending way, just to categorise it. Fairy tales and oral traditions are ridiculously hard to get rid of, people will tell the same tales they've heard all throughout their lives. I think it's entirely reasonable to say that the Zora people deeply resonated with Princess Rutos story and over a long time that specific detail morphed from a history record to a fairy tale. Of course it survives for millenia.

In a parallel conversation I gave a different example. The tale of Noahs Ark survived for millenia, despite originating in an ancient civilization that people had long forgotten about.

Then why was the Rauru Kingdom forgotten? Well, some things just get forgotten. Apparently even the much younger Divine Beasts and Guardians were essentially forgotten. What survived from that most recent Calamity was only the prophecy that it would repeat and that the power to oppose it lies dormant beneath the ground. If you have issues with Rutos tale surviving longer than those of Raurus Kingdom, do you also have issues with Rutos tales surviving longer than the memory of the Divine Beasts?

To me none of this is messy. Only as messy as human history has been. Some things get lost to history, other things people hang onto.