Counterargument: series that introduces second male lead late makes me not want to root for them. I already got used to the first male lead and is actively supporting so suddenly jumping ship is.... I mean if the first male lead was a glorified douche bag and comparatively the second was a nice guy, then yeah I'm fine with that but those guys are most likely to get rejected.
I think it's not about running out of ideas but rather the relevance of the character's appearance at that point in time of the story. Like to be a rival to mmc/fmc, then pushes the other to be more open with their feelings. Sometimes they're the reason why fmc and mmc met in the first place like in skip beat! with sho.
Tbh I agree with you that author did introduce the second male lead early. The pace of the story is kinda fast. I didn't realise it before you mentioned it since I was reading the manga by release. But imo the shoujo mangas that usually ends in like 3 volumes are better than ones that unnecessarily build up drama to make it longer and also making me hate the mmc in the process.
DEFINITELY agree with the shorter shoujos being better. There are some good longer ones, but more often than not they're frustrating. My problem is probably that I just hate love triangles. I don't want the second male lead introduced too early because I really don't want a second male lead introduced at all if possible...
I call it running out of ideas because a lot of authors treat the 2nd ml like a deus ex machina, as in they can't figure out how to make the characters develop their relationship further or have written them in such a way that it's impossible to move forward so they throw in a new character to give them that little push. Which is exactly what you said but I've read one too many series where the added conflict was rushed and unnatural and wound up ruining the pace of the story and becoming extremely frustrating.
This case is different because the 2nd ml is introduced way before we hit any road blocks, but it's still too early since the fmc and mmc have barely had enough time to talk to each other before the new guy comes in. And he's already made a terrible impression so at least to me his only purpose will be to slow the story down and just be a pain in general.