I understand that yuni is a piece of shit and everything she spits are nothing but excuses but she's been really transparent on what she wanted from their relationship from the very beginning.
This is not true Yuni has not once said these things to Nanase. Communication is important. Writing stuff on twitter does not count. Furthermore, Nanase asked several times, giving Yuni the opportunity to express herself.
yeah like a big part of the earlier chapters is Yuni literally just thinking to herself how she wanted to say something but didn't because she either didn't dare or because she was testing her relationship in ways she herself realised she shouldn't be. A la her not saying anything about the anniversary until it was already over, or her not saying anything when Nanase kept choosing the club over her.
In that way Nanase certainly isn't a good girlfriend but also she could not have known that Yuni felt lonely because Yuni wouldn't tell her, now this isn't really Yuni's fault either because this is clearly both of theirs first relationship and you just make those kinds of mistakes through inexperience. Honestly their relationship would most likely have been fine if Fuuko didn't come along and seduced Yuni. In that case Yuni would have most likely eventually started talking about her problems and Nanase would have followed up on it.
Though then again even though Fuuko is definitely the reason this relationship is on the cliffs I still wouldn't say she's evil because she clearly has issues she needs to talk to therapist about because she never got over her crush on her neighbor who got married to a man, and is as a result just desperately trying to make herself needed to someone even if/especially so she can be their second fiddle.