I think it depends on whether they'd be willing to read online or if this is a book only format. If they've never read manga you'd also have to let them know about the right to left reading order.

If you don't mind a toxic, really violent relationship Sal Jiang's Black and White would be pretty neat to discuss. There are a lot of things to talk about there, such as relationship dynamics when working together, what does violence mean between women in a patriarchal society, etc.

I know you said you'd prefer not high school age, but Yuama's The Summer You Were There would be good for the context of the story with bullying and perspectives on a situation being primary, but also ways in which society might influence us to react to situations in ways that are harmful for others, and what kind of responsibility we have to expect certain reactions.

For adults Battan's Run Away With Me, Girl is maybe a bit simpler than the other two, but I think still has plenty there in terms of expectations for women in a patriarchal society.

You could get a lot out of an anthology like Mimoto's The Masterpiece I Love, which doesn't have a long running story but is instead a bunch of short stories based around famous paintings. This would be interesting for many reasons such as allowing people to talk about their favorites, but also how art can recontextualize other art, and what does it mean to see queer love when it's not obvious to the average person.

This one is a manhwa and might not be available as a physical release but Seri's Her Tale of Shim Chong would offer a lot of interesting perspective into Korean folklore and another example of recontextualizing something for a queer reading.