I had the same question, my comic is sci-fi but also the entire main cast is some flavour of queer, the main character transitions in the story, and a lot of the narrative is reflective of a queer experience, even though it isn't necessarily specific to a queer experience. Stuff like found family, colourful misfits, fighting for rights, not feeling right in your own body, keeping secrets from loved ones, not being seen for who you are, outcasts by choice because it's easier to live on the fringe of society authentically than to conform within it... Some mixes of those things will make a story feel like a queer story because it speaks to queer experiences.
I made use of the multiple category system personnally, my comic is still sci-fi drama first and foremost, but it does have a lot of queer themes and queer characters who, yes are sci-fi protagonists and antagonists first, but they're also queer and that's a part of the story that can't be erased without taking out a substancial chunk of the story. Khay is a qatari refugee in France while France and Qatar are at war, she's stubborn and stuck up and mistrustful of those around her, but she has a fundamental kindness and moral compass that she can't seem to shake. She's also trans and comes to terms with her gender and able to become a better person, more comfortable with herself and more open with new friends as the story progresses through them accepting her and helping her even when she's vulnerable. Take her transness out of the story and there's no catalyst for her to open up and learn to trust her found family. Her character arc goes out the window.
That'd be my barometer for if your story should be classed (or sub classed) as lgbtq+:
- Would it be possible to take away queerness without fundamentally changing large parts of the story?
If yes, all the characters could be cis straight people and nothing else would need to change, then no tag. If not, depending on how much the story would change I'd put it as a tag in first, second or third place.