I can sorta understand the distaste for Milo and Mal's relationship because it doesn't really show that sweet childhood "friends" to lovers trope in the traditional sense. There's also of course the "issue" of night market entering the relationship, the whole complaint over their preexisting history and how it feels the night market is third wheeling on their relationship. That one I find less understanding in, since they do have a long history before the story begins for the night market enters(something I quite love, it's really cool when characters' back stories are fleshed out), and one's previous relationship and experiences doesn't prevent them from creating new ones, even if the new ones are built off/around/on previous ones. That's life.
Now, personally, I don't feel that way at all. I'm someone who has lived and continues to live in a similar limbo-esque relationship. While my experience obviously differs, it follows a very similar timeline. We knew eachother as children and grew up around the other, had some weird not-quite-relationship for years before delving into a labeled one, which went to shit after some time since we both had (and probably have) many, many issues. A lot happened in between, but it was always weird and complicated and hard to explain. We knew each other at some of our best and worst moments, when push came to shove we'd go to the other for some semblance of comfort. I probably love them as much as I hate them, I don't think I could live without them but we didn't do great together at the time. Long term relationships, of any kind, especially when they started in childhood are always complicated and messy, and I just really like seeing that side of it within media.
You should write their relationship as you want, I think the complicated and always entwined tone to it is beautiful and represents their relationship and others like it very well. And this goes without saying, but your story is amazing, all the characters have my heart. Thank you for writing.
I have never understood the rage about romance options having a past. It's been such a weird thing to see because I have always felt like it makes the characters fuller. Especially in a story as big as thing? They were born before the story began and it needs to feel like it. I, like you, adore the messy.
There is something that has really really helped me with my writing and how I view people liking/not liking my work or choices. I am in general, never a part of fandom spaces. I'm just not someone who has every done the community thing when I like a show or a book or a movie. I will gush about it to the people in my life but I'm not really interacting with people online about it.
There are a few fandoms that I stumbled into, however, due to just algorithm. I stuck around for a while, enjoying the kind of joy that came from it in the beginning. But then, like everything, it became toxic as time went on. Cancel culture started up. People got online and started freaking out about things that have no baring on their life. Actors got death threats. People would take one miniscule situation and not look at the nuance or cultural impact that it would have. Etc.
Now, that is all on a BIG scale because these are popular fandoms. Night Market doesn't even touch those. However, I deal with all the same kind of vitriol at times. I don't know why but it left me actually able to deal with some of the more negative aspects of this better and stick to my own version of what I wanted to write, mostly because it is clear that no matter what, someone will always find fault in my work. Someone will always look at what I do and disagree to such a point that is disproportionate to what I thought they would do.
That being said, that's all okay. Everyone is allowed to do what they do and form their own opinions. I know my characters though and what I want to write and at the end of the day, that's the voice that is going to matter to me the most. It's a strange thing putting yourself first when it comes to public consumption and I think most creators need to do it more often.