Honestly, I don't think this manga needs an introduction so i'll keep it short; art if great (above-average for a modern manga) Gojou and Marin are believable characters.
The story up to the last arc was enjoyable and fresh. However, in the latest arc (the one with nuns cosplay), things sort of slowed down and got focused a bit too much on cosplay and - honestly - got stale.
yes, this manga was always (officially) about cosplay, but it was a vehicle which set up and moved the plot, and that plot was Gojou and Marin's interaction and Gojou's growth and socialization.
This has made the manga interested for audience who weren't into cosplay per se, but wanted a good story.
But this latest arc focuses on cosplay so hard and so much and so thoroughly that i feel it lost it's freshness.
There might be a reason for it tho. i believe that plot-wise the manga has reached a level where it has sort of painted itself int the corner. Namely, Marin understood she likes Gojou (who's annoyingly dense A F about everything). there's only one proper way forward, but this would at this point mean effective end of the manga, so i suppose the manga-ka didn't know what to do so just started hammering on the cosplay thing HARD.
Which is a shame, because I think here ARE things that need to happen. For example, make Gojou grow.
I have a problem with Wakana Gojou as a character.
OK, he's introverted and antisocial and had a trauma, OK I get all that. And this is all good as basic setup from which growth is to happen. But in my opinion he still doesn't really grow.
For example he is still under sway of that ONE SINGLE incident from his childhood, even though there's no-one ever who says anything negative about hina dolls as the manga progresses.
in fact, no-one ever looks down on him for any reason. on the contrary, there are many people who respect him and consider him important, and none more than the brightest-shining girl in the school!
So how he STILL chooses to be under the sway of one single incident caused by one brat from x years ago is beyond me.
And it's also kinda disrespectful to people around him who respect and love him, actually. All the time spent with them, all the things done, and that screaming brat from his childhood STILL tips the scales? come on, man!
secondly, he is a healthy male. we know this because... we have seen him. And that's OK. We know he is attracted to Marin in THAT way. That's also OK.
so, how HE is not in love with (or at least romantically aware of) Marin as a (very pretty) girl is kinda... hard to explain. But if i tried to find a reason for it, i think the reason would be that his self-esteem is still so low in the gutter that he can't dare to even like her. Which is, again, a rather pathetic response which shows zero growth on the inside.
And all of this would be OK as starting something he improves on. But, it doesn't get addressed, which is a shame. so, instead of cosplay boot camp, perhaps a Gojou personal growth arc wouldn't be bad.