My advice here would be: don't think in terms of "right" and "wrong"; think in terms of structural arcs. You need to figure out what options for arcs you have, and which of those you actually want to use.
You've basically presented three possible arcs:
- The primary arc, where in some meaningful way, the primary plot is about the protagonist's homosexuality, or profoundly connected to it. (This is a direction you're not interested in at the moment.)
- Upfront disclosure; no arc, where we know right off the bat that the protagonist is lesbian, and it's largely treated as a minor point for the rest of the story.
- A "gradual reveal" arc, where the character's homosexuality is initially unknown, and effectively "secret"; we get hints and clues
along the way; and at some point we reach the "revelation" that the
character is lesbian.
The advantage of the "upfront disclosure" arc is that it gets you closest to your goal of your protagonist's sexuality being matter-of-fact, rather than making a big deal over "OMG she's a lesbian."
The disadvantage is that her sexuality can feel like an informed attribute, where you declare her to be a lesbian but that doesn't actually seem to come into play in any way, so the declaration feels artificial, or pointless, or like bald tokenism. In general, there's a popular assumption that the "default character" is straight, and that diverging from the default should only be done if the divergence makes some significant impact on the story. (That's a big discussion which I don't have scope to go into here, but it stems both from fiction's tendency towards "conservation of details" - i.e. "significant details will prove to be important; otherwise you wouldn't put them in in the first place" - and from popular social mores, by which most people consider a character being homosexual as a "significant detail," while considering heterosexuality to be a sort of unremarkable, default expectation.)
With the "gradual reveal" arc, your advantage is that by turning the attribute into a surprise, you've created an arc around it. It's in focus. The shape of the arc will force you to substantiate your protagonist's sexuality, so it won't feel like a minor detail or an aside; placing the revelation as the culmination of an arc makes it feel important.
And the disadvantage is that you lose these sense of her being a lesbian as immediate established fact, as not exceptional or surprising. In many ways you might be undermining your protagonist's lesbian identity; if we can follow along with her for chapters or entire books without it ever coming up, then how important or significant can it be? It basically locks you into bringing in her sexuality in one very specific, fairly limited way.
If you understand the advantages and disadvantages of both, then you can choose one, and compensate. If you want to deal with the risk of "informed homosexuality," you compensate by finding other ways to make her sexuality evident and significant. If you want to deal with the disadvantages of the gradual reveal, you compensate by making the structure work and make sense, giving weight and meaning you want to your protagonist's sexuality, not just whatever fits most easily into an arc. And so on.
But there's another alternative for you to consider, which could mesh well with either type of arc, or do the job on its own: consider a sideplot. You're mentioning multiple volumes, so you've got lots of room for secondary characters and sideplots. In that case, what you can do is establish a sideplot, which forms its own, more minor arc. And this arc has the same advantages that a primary arc has - it lets you put some focus on your protagonist's sexuality, it lets you make it important, and it lets you choose what's important about it, and how you bring it into play.
I don't know your character or your setting, but you can come up with lots of ideas. A community of queer friends; a male ex from when your protagonist was coming to terms with herself; an arc about the kind of family your protag would like to have someday; a queer relationship between secondary characters that your protagonist is privy to. Any one of these could be the basis for a solid, secondary plot, which wouldn't overwhelm the main story, but would come up and recur.
Another big advantage here is that a subplot lets you figure out some aspects of homosexual life that you're interested in spotlighting; being a lesbian affects a person's life far more than just "what gender is my partner", and it sounds like that one question isn't what you're interested in with this character. So consider: how is your character's life shaped or affected by her sexuality? Romance, relationships, community, worldview, social position, something else? What there interests you?
When you look at it this way, I think you'll find that the "gradual reveal" arc is a subplot - it's just a specific one, climaxing with the revelation that the protagonist's sexuality exists. But if that isn't the subplot that you want, you just need to choose a different one you like better.
Hope this helps. This can be a delicate issue, and you're swimming against some deeply-ingrained biases and expectations. So it makes sense that it be difficult, and feel unnatural and unbalanced.
One approach is to say "What the heck," and accept a balance that feels less than perfect - erring on the side of "informed homosexuality," or using a simple "revelation: lesbian" arc. These can feel unsatisfying, but they're valued by many, not least because they're pushing against that deeply-ingrained default.
If you're able to put in more effort and find something you feel really works, I think that's even better :)