@quixoticgeek Legends and Lattes fits at least most of these requirements I think. I loved it, though I listed to it on audible. I’m not sure if you can buy it in ebook without DRM or not.

@quixoticgeek Hm, I'd recommend my books but wrong gender, sorry.

@quixoticgeek The Long Way to a Small Angry Planet by Becky Chambers fits all of these (I think). It’s part of a larger series but each book is completely stand alone, and this is the first one anyway. I often describe it as wholesome sci fi. It’s available on bookshop.org but not positive on the DRM-ness of it (I read it through my library), which is the only thing I’m not certain about: https://bookshop.org/p/books/the-long-way-to-a-small-angry-planet-becky-chambers/6437435?ean=9780062444127&digital=t

@quixoticgeek shroud by Adrian Tchaikovsky fits 5/6 of those. It's not especially cheerful. One of the main characters is at least partially sapphic. Might be bi. It's part of the interpersonal relationships of the crew and isn't about them being that, hence why it didn't stand out enough for me to remember.
Tchaikovsky does really interesting alien encounter stories with huge depth. If you've read 'children of...' series it's similar in topic but different in scope.

@quixoticgeek Have you read any @aliettedb? Sounds like her "lesbian space pirates in a future Vietnamese empire" stuff might be up your alley. Some are in the same universe; others are standalone. Most have a romance element, but they're not Romance. And, crucially, I always finish a book feeling good!. "Navigational Entanglements" might be a good start because it's a standalone. And they're published by Tor, so available without DRM through e.g. Kobo.

@quixoticgeek The Priory of the Orange Tree

Should add. Any book that's part of a series, the series needs to be complete. I don't want to start another series and be left hanging waiting for a conclusion.

@quixoticgeek @forth The Mimicking of Known Successes, perhaps? lesbian sherlock and watson in space (complete with victorian-era setting tropes like train travel and gaslamp lighting)

only point it may not meet is the cheerful/happy/positive? i didn’t find it a downer of a read by a long shot and i’d say it averages on the happier side of things but it’s not like the point of the book is optimism either. i would say it’s more “cozy” than “cheerful”.

@quixoticgeek The Locked Tomb series might fit the bill? Initially intended to be a trilogy but an extra book snuck in a #3. Book four is expected in a year or three.

@quixoticgeek Have you read The Long Way To A Small Angry Planet? It may not meet quite all of your criteria (and I specifically can't speak to the last one) but I suspect it hits the general feel. It's... cozy.
(and while it is arguably the first in a series, that doesn't matter - the series is only loosely connected)

I also recommend all of the Parasol Protectorate series, starting with Soulless. Technically there's romance, and mostly not involving lesbians, but the overall theme is "silly".

@quixoticgeek The Unstoppable Trilogy by Charlie Jane Anders is YA SF with sapphic characters. It has some dark moments, but is ultimately hopeful. Has multiple romantic subplots, but I wouldn't say that's the genre.

Same is true of her more recent Lessons in Magic and Disaster, but it isn't YA and the dark moments hit a bit harder for me; I'd say it's more an uplifiting sort of hopeful than a cheerful sort, so maybe not quite what you're after?

(I feel like I should be able to come up with more, but most of my sapphic reading to date has been romance.)

@quixoticgeek I was all set to suggest Too Much Lip by Melissa Lucashenko, but it doesn't seem to be available DRM free (I read it via the library) which is a shame.

@quixoticgeek Ah, hah... my main MLP fanfiction I'm working on right now ticks three of those. Four if you count freely downloadable chapters.
Once done it should have five parts, but I'm working on finishing the third right now. I can't promise they're all good...