Two paragraphs might not be an essay, but it sure starts like that. You've already thought their relationship through and came to what we could call an abstract. All that's missing is... a more detailed explanation and some citations.
The only ones of the core cast that even have a relationship other than Mamoru and Usagi are Haruka and Michiru, and they clearly have a much better relationship than the former two, xenophobic as they are at times.
Secondary characters, well, there are Usagi's parents and Naru with her Umino. They seem to be happy, but they are barely around after R.
Rei with Mamoru was odd, and it always seemed Mamoru dated her because he had nothing else to do, not because he seemed to be very interested. For a while I thought Rei did that to annoy Usagi, but she seemed to have really cared, with her offering the temple for him to stay at after she ruined his room and being really annoyed at Usagi for her insinuating that she might try something (and also, like everything in this utter waste of a season, Diana goes and makes it SUPER CREEPY by offering to be Mamoru's chaperone, insinuating that she expects Rei to hop on top of him the moment they're alone).
That leaves a lot of other side characters that never went anywhere, like Yuichiro, Makoto's Kenji and Ryo. I'm not sure why Ryo was dropped*. He knew Ami was Sailor Mercury, they shared common interests and Ami never expressed anything more for someone else. Except fangirling after composers and the Three Lights.
I also just made the mistake of googling a bit for any other side characters I might have missed, and found a list of fan-shipped couples that could have been. I want to scour the Earth of people who think that any of the Inner Senshi in the anime are supposedly coded to be lesbian or have lesbian subtext (there is subtext in the manga for Rei and Minako). They don't and aren't guys, please stop that.
Chibi-Usa has two potential relationships, one of which I don't want to talk about, and the other is Hotaru, which requires way too much mind-bending to make it work.
I recently came up with an idea for one last examination post, and that's character arcs beyond Usagi. The short gist is that, if we allow the SuperS power up episodes to stand there while ignoring the rest of the season, we can reasonably conclude that almost everyone's arc is kind of closed... except Makoto and her senpai. It would have been really nice to give Makoto an episode in Sailor Stars where she finally frees herself of that. Sure it made for a fun running gag (and I answered one of my earlier questions if Makoto and Ami ever talked to each other alone - yes, they did, in early R when Makoto talks about how Ali/Ail could be the Moonlight Knight and Ami asks if he reminds her of her lost love. Makoto initially says no, but then says both love music, and Ami just falls over in embarrassment). So they do talk to each other, but they fail the Bechdel test in the process.
So yes, two actual couples in the core cast, two side couples that don't really show up after R, and lots of "what if?"s. For a show about five girls fighting for love and justice, that's not much, is it?
There is a non-canon manga side story where the daughters of Ami, Rei, Makoto and Minako try to befriend Usagi's second child. I'm going with that. Yep.
*I mean, in-universe. It's perfectly understandable why the writers dropped that.