lol. lmao even. "First few chapters" he said. Romance are one of the main plot of the manga bro. It's there from the start until now and will still be there in the future. I could even go further to say that the romance parts are the best highlight of the manga for casual reader of this manga.
The author barely show any actual cosplay making, and just show some bit of it like making props, and some trivia here and there like how to wash wigs. There's no step by step detail on how to do all that at all in this manga.
Hardcore fans have hard time differenting setting from main story, especially when you'll start criticize their favourites. Cosplay was always setting, it was never main story, even now. Acquaintance between Gojou and Marin was always main dish, cosplay was only mean to connect them. You can literally see it the first chapter, where both of them are introduced and meet each other before all that cosplay stuff comes up. It is glue, that holds this story. It was at the beginnig and it is now. Nothing changed. But somehow, some people will still argue about that.
And people say "iTs a CoSPLay foCuS mAngA!". This is the stupidest argument I've ever see. This manga was popular because of Marin and Gojo romance interaction. That was what bring most people in. There's no way it'll be as popular as it is now if it's just the cosplay. So when the author did this shit for like 10 chapters, in a monthly manga magazine, with a very few pages each chapter for a "Monthly Manga", of course most of them would be upset, even for someone who follow this serie since its first chapter was out.
Don't even try explaining it. You can literally link each chapter from first half of the manga with brief summarization of that chapter and list every aspect that is main focus of such (which are not only romance, but overall interactions between characters, ecchi, Marin's persona, personal development of main characters, development of the story etc.) and show that there was much, much more than just cosplay, you can explain over and over, that the problem is not only lack of something, but rather an overwhelming amount of something else, you can explain, that release schedule is not working for that kind of slow storytelling, that the change in release schedule isn't compensated neither with quality nor with quantity. I'd even go that far and state, that story in Gojou's arc is not even good; author shows him rather as a maniac and not a perfectionist, which we've seen at some point, so it's not only repetitive, this arc isn't even good at developing his character by such.
All you'll get is, as you said,
"iTs a CoSPLay foCuS mAngA!".
It is as someone would suggest after reading arc about those 2 sisters (loli and overgrown middle-schooler one), that this is photography focus manga.