I've always wondered but are Hotaru and Saturn two different people? That was always the vibe I got from watching Stars.
Here's how I figure it works.
Princess Serenity and Prince Endymion (for example) lived in the past and grew into teenagers, met and fell in love, and then they tragically died. But Queen Serenity rigged it so their souls would be reborn again to have a second chance. So Serenity and Endymion were reborn to entirely new families as Usagi and Mamoru.
Through Season 1, they met and fell in love for a second time, and their powers activated, but even as Sailor Moon, Usagi always remained Usagi. Over the course of S1, Usagi and Mamoru's memories slowly "awakened" and they remembered their past as Serenity and Endymion, but they never stopped being Usagi and Mamoru (aside from an occasional brainwashing or whatever).
This is because Usagi
is Serenity, and Mamoru
is Endymion. In this universe, the soul exists, and in the argument of nature vs nurture, nature wins. Serenity may have been raised a princess and Usagi was raised a commoner, but Usagi will always be Usagi, and always
has been Usagi, even all the way back when she was known by the name of Serenity.
As Serenity, she remembered being born to Queen Serenity and raised to be a Princess, and as Usagi, she remembered being born to Mrs Tsukino and being raised to get good grades. But after her memories awakened, she remembered that she had both of those experiences, and the experience doesn't really change her, because she was the same person both times. Two different-looking paths aligned and merged, but they were both fundamentally the same path all along.
Sailor Saturn was reborn to Professor Tomoe as "Hotaru", and seems to have had a radically different upbringing (although we don't know anything about Sailor Saturn's past, so maybe not?), so maybe the awakening of Saturn's memories in Sailormoon S changed Hotaru as a person, but I doubt it. Deep down, Hotaru was always Sailor Saturn reborn. And then she died and was reborn yet again.
In Stars, baby Hotaru is nowhere near equal to the development of Sailor Saturn or the previous Hotaru, so Sailor Saturn's memories are "asleep" to allow her to grow up normally (I believe at this point there's no distinction between the sleeping Sailor Saturn and the previous incarnation of Hotaru, because Hotaru remembered her past as Sailor Saturn, and Sailor Saturn (while sleeping) experienced a second life as Hotaru, so the two are one and the same). But external forces required a rude awakening of Sailor Saturn, who is clearly different from a baby, and whose emergence even required a magical body growth. Baby Hotaru's body can't even
handle the stuff inside Sailor Saturn's head (but that's okay, because Sailor Saturn can fix that).
I don't think it's at all clear what kind of person Hotaru V2 is by herself, like if Sailor Saturn went back to sleep, but... them's the breaks. Plans get messed up. And it's all Sailor Saturn in the end anyways. She only has one soul.