I would argue that, on the contrary, Michiru receives tremendous development in the 90s anime just that... it happens before S.
A major part of her development is featured in #106.
While at first glance it majorly focuses on Haruka, it's also a lot about Michiru.
As we discover, Michiru is regarded as the perfect Japanese girl - feminine, classy, polite and artistic.
However, she is in fact a very solitary girl with people regularly making comments on how she hates people.
On the cruise ship, she performs a very sad and foreboding music and even paints a giant piece depicting the end of the world.
While her mindset definitely went gloomy after she awoke by her own as a Senshi and she started to have premonitions of the world ending, it's safe to presume even before that she was lonely and wasn't much interested in dealing with people.
However, once she falls in love with Haruka, she finds an anchor in that and even more so when she discovers that Haruka is her destined mate, she sees a glimmer of hope.
We also see that before Haruka reciprocated her feelings, she was far away from the confidant girl she is in S - when she first meets Haruka (whom she's been in love for so long), her whole body shakes and she can't find her words, on the cruise ship she is visibly affected by the comments of the passengers badmouthing her and later when Haruka and her discuss in private, she dances around the subject of her feelings commenting there's a supposed girl at her school who wants to go on a drive with Haruka.
When Haruka rejects her, Haruka breaks her heart as well as her soul.
Haruka: If someone has got to do it, then just go and do it by yourself! I'd like you to stop putting your nose into my business!
Michiru: Don't say such selfish things... I, too, don't want to do it. Even I, I have a dream of becoming a violinist... I can't keep doing something as rubbishly stupid as saving the world from devastation!!!
It's at this point that Michiru gives up on both her love for Haruka and as well as having a normal life and as we see in their next encounter, Michiru paints herself as a cold-blooded monster ready to do whatever it takes to do her mission.
Her development and characterization are largely reflected into her songs of S,
Ai Dake - which depicts her feelings of when she fell in love with Haruka and how that brought her solace in her dark life.
The loneliness that is being lit upon inside my heart is,
The foreboding of a budding love,
This feeling that can barely wait for tomorrow,
Adds up to one another of my secrets,
Whenever we feel love towards someone,
It conjures up a mysterious strength,
Whispering your name that I still don't know,
Is my incantation that I'll forever protect you know,
The passions of my heart are even now still having dreams,
There's some things that only love can do,
Thus was the premonition I felt.
I'm sure there will be a tomorrow where we'll look at each other with a passionate gaze,
May you guide me without fail and from then will a strength be born.
Since forever, they have been whispering to me,
The ghosts of my heart,
There's still something that love aims to do,
The future is opening forward!
Even now, the passions of my heart are having dreams,
There's something that only love can do,
That is my prophecy!
Unmei wa Utsukushiku (Destiny is Beautiful), depicting that if Haruka returns her feelings, then Michiru will be able to fight whatever destiny throws at her.
It's basically a confession song of Michiru to Haruka and in turn a companion piece to Ogata's Shiokaze ni Nosete.
When I first saw you, at that time,
That nostalgic smile of yours felt like the scent of the wind,
Called upon by a mysterious force,
I immediately realized that's how I got to meet you,
Beneath this vast sky, I was forever searching for those passionate eyes of yours,
This destiny of mine is oh so beautiful,
For it's making my tears shine (she cried tears of joy when she realized she was in love with her soul mate~)
Whatever memories might shackle me down,
I felt like I would now never look back.
The fingertip that you held out to me,
Is slowly being dyed into a deep color of darkness,
Even if that faraway dream of mine of waking up in a cold storm may one day come true,
Believing into everything I've chosen, will give me the power to change tomorrow won't it? (So, loving Haruka is her greatest strength)
This destiny of mine is oh so heartwarming,
For it's making my skin shine,
Whatever sadness may close in on to me,
If you're by my side, I won't ever be hurt again.
This destiny of mine is oh so beautiful,
For it's making my tears shine!
Whatever sadness may close in on to me,
If you're by my side, I won't ever be hurt again,
If you're with me,
If you're with me...
And we do see this is a constant in Stars when Michiru several times, proclaims Haruka as her rock.
Especially in her death scene.
Michiru: If you're with me, I can even endure being burnt in hell.
Haruka: Hell? It doesn't suit you, you know.
Michiru: I don't regret it. (Has so much meaning after Unmei wa Ustukushiku)
Michiru's character is pretty much complete once Haruka returns her love - she is no more lonely, doesn't care about what others think (in Stars, we see she has become more and more social and makes jokes and flirts openly with Haruka), even her music has changed (again as we see with her joint performance with the Three Lights) - also, she didn't give up on her violinist dream! and for the whole of S, she isn't affected by the weight of her premonitions as she previously was.
At several intervals, she asks Haruka to join hands with the girls with the latter refusing because Haruka refuses to force this burden on others.
While Haruka, who appears so strong on the outside, is deeply traumatized by their destiny and depresses over having to sully her hands,
Michiru, on the other hand who appears so refine on the outside, is much more stronger than her because of Haruka.
Michiru's only weakness is Haruka.
While Haruka is ready to sacrifice any innocent for the greater good, Michiru is ready to sacrifice the greater good for Haruka.
Michiru has demonstrated that she'll readily sacrifice herself or the world if it means Haruka is saved, afterall her whole world is Haruka.
They are really like fire and water.
After the Grail appears, she doesn't have much stuff to do but is complementary to Haruka and proves that she knows her better than anyone else.
#118, 'You know, Setsuna-san, Haruka here believes in Sailor Moon more than any of us.'
Sure, it would have been cool if she had a focus episode but that's pretty much the SuperS special?
In hindsight, she is like a Manga!Mamoru done right.
In the manga, however, I'll never understand why people love that ONE scene of Stars so much.
It's just so OOC for me.
I much better prefer her in Dreams/Eternal getting flustered over Hotaru breaking her expensive china - Michiru being a rich and artiste girl, it'd make sense that she has some expensive stuff at her manor and breaking them is well, disastrous!
In Stars, though, she being triggered over lipstick?!!?
I think that Takeuchi mentioned in the data books that Michiru collects cosmetics (but who cares we NEVER see that in the manga), still isn't she supposed to be super mature and etc.
Takeuchi just turns Michiru for that one panel into the standard vain Ojou-sama character that every shoujo manga has and like always Takeuchi's jokes suck.
If it was 90s Michiru, she'd be unfazed and probably comment something like, 'Ara, ara. The young boys of today sure have a tasteful fashion of having a broomstick glued with duck tape on their head desuwa ne ~'. (smacks lips)