

Part of the fun of DMing for me is in homebrewing cultures…or, more accurately, homebrewing factions that have a culture.
Besides which, there are some fundamental flaws in your premises:
You assert that a counterpart culture is easier to understand than an original one. I 100% understand any culture I make up, definitionally. On the other hand, neither I nor anyone else at my table can say the same about any IRL culture. Even members of a given IRL culture can never fully understand the totality of it.
You also say
[if] you create fantasy ancestries from scratch, you need to convey all that information to the players.
And I don’t think that’s true. Players don’t need to know everything about a culture to interact with them. In many cases, the player characters are themselves unfamiliar with that culture, in which case any mystery, mistakes, miscommunications etc are valuable in-character roleplay. And when the PCs would be familiar with a relevant aspect of a given culture, you can simply tell them that detail, no need to loredump everything. (Eg “I beg for mercy” “Your character knows that The Southern Pirates are notorious for never taking prisoners, are you sure you want to try that?”)





No, of course not. Why would anyone waste effort on infinite irrelevant details? But everything there is to know, I know.
You give them an overview at the start with the information you guess might be relevant or interesting to them, and supplement it during the game as necessary.