Honestly, Stars would have been so much more interesting, IMO, if instead of the Starlights showing up and the Inners and Outers getting picked off two at a time, Sailor Kakyu had shown up on Earth seeking allies to help her defeat Sailor Galaxia. Then we could have had the characters we already know split up into two teams: one team would go with Luna and Artemis to check on their home planet and gather allies, the other would go with Phobos and Deimos to their home planet to do the same. That way those ideas could have been sufficiently fleshed out, we could have seen what life is like on other planets, and the Inners and Outers would have gotten plenty of time to shine.
Maybe their actual forms are humanoid cats on Mau. That could be why they prefer cat forms on Earth, which is more similar to their actual forms. And simply being a cat as compared with a human being makes it easier to blend in and avoid attention.
Maybe they chose to live on the Moon for work, kind of like how people to overseas as part of their jobs. Or they could have simply migrated to our solar system for some reason.
There were a lot of things in the Stars arc of the manga that just felt like Naoko was tossing in new ideas without ever going into them to have them explored any further, as though she hoped that these ideas would entice her readers enough to make them want to keep reading all the way to the end, instead of actually doing some world-building with them. The cats being humanoids from the Planet Mau was one such concept that she threw in without further exploration, and kinda feels like it goes against the miraculous nature of their transformations into human form from earlier in the story.
When Luna became human in the Princess Kaguya sidestory, it was treated like a sort of miracle brought about by the Silver Crystal. Then, when the same happened again for Artemis in the Dream arc and then for all three at the Dream arc's end, it felt like it was done to further illustrate the wonder and momentousness of those occurrences, again brought on by the awesome power of the crystals (plural, as these times involved more than just Usagi's).
But then when the Stars arc comes along and tells us that the cats were actually humanoid alien cat people the whole time, it kinda cheapened the majesty of the cats' previous transformations into human form, as they were no longer viewed as taking on all new forms, but instead reverting to forms that they were supposed to have had all along, but couldn't assume anymore for reasons never disclosed in-universe, when the real world reason was because Naoko had originally intended for the cats to be cats and only just then made up a new backstory for them; a backstory that ultimately went nowhere and amounted to little more than backstory piling for the sake of backstory piling.
And I get why she didn't, or couldn't, explore this backstory any further. The manga was heading to its end. She likely didn't have room to go into it any further than she did. She obviously wanted to give Sailor Tin Nyanko some kind of compelling reason to make readers interested in her, and so thought up a basic backstory idea that would tie her in with the cats, familiar characters that the readers had followed since the beginning and thus could be used as a hook to pull readers in further to make Sailor Tin Nyanko, the latest disposable villain lackey, seem more interesting than she actually was.