Ohmygosh Hi! I’m really flattered that you want to know  more! I absolutely don’t mind sharing my thoughts, since I feel like with  SilMil fics, the fun is in the details rather than the broad strokes.
I have to preface this by saying that I’m terrible and mix canons for my
headcanon, so I tend to end up going with manga/new myu SilMil but an
anime-based 20^thcentury.
I see SilMil Uranus and Neptune being a lot like Haruka and Michiru, except
Uranus is even more headstrong and reckless and Neptune adheres absolutely to
her duty. Uranus needs people and feels like having this power means she needs to
protect people, and there haven’t been any outside threats for god knows how
many years, so why should she stay in her castle so far away from everyone? She
makes visits to the Moon often enough that she’s nearly as close to Serenity as
her guardians are. Venus hates her
for it. She has literally one job, and she’s not doing it. But she also envies her ability to disregard all she’s supposed to be, so she can’t
bring herself to make her stop (she absolutely could make Uranus toe the line,
with physical/magical force or with disciplinary action through Queen
Serenity).
Neptune, meanwhile, does stay alone in her castle, with only her mirror for
company. The mirror could show her a lot of things, but like a good soldier,
she only uses it for her duty. She looks for intruders from outside the solar
system.
But the war between Moon and Earth is coming, and without the Silver Crystal,
there will be nothing to draw threats for thousands of years. So what she sees
are flashes of the Death Busters. Maybe Galaxia on occasion. And she sees
herself fighting them, alongside the woman she’ll love. She doesn’t know when
they’ll meet, but they have the same duty, so surely, someday, they’ll be
together.
Then the War happens. Neptune feels when the first senshi dies, and then
another. She does not leave her post, her duty was drilled into her, she will
stay. But when she feels a third senshi die, she casts her mirror to watch.
Uranus is with them, fighting. Uranus is with them, dying. She sees it this
time along with feeling it.
Serenity dies, and then the Queen.
Neptune’s duty means nothing now.  It
never meant anything, her kingdom burned and she stood by. Any future she’d hoped for was gone.
She leaves for the first time since she was stationed. She doesn’t expect to
find anyone alive on the moon, but she finds Saturn.