Okay a couple things.
Didn't bts not have solo insta accounts until recently? And even with the account a couple members haven't even been using it. Instagram isn't really doing much for a well established group. BTS still got the LV deal without insta.
About solo promotions, I do agree that twice should have been promoted solo back in 2016-2017 when they were the it group in SK. But I don't think they will leave JYP and go solo. Twice, the name, is bigger than the individual members. If you look at got7 jackson, bambam, and Jinyoung are the only members that seem to have successful solo careers since they were the most popular members. I think momo and tzuyu and maybe 2 other members will have successful solo careers but that's about it.
All in all, I don't think jyp was necessarily poorly managing them, I think the company was focusing more on he ot9 agenda like bighit and bts. Even in bts, the rapline has their mixtapes but that's about it. And those weren't even promoted by the company that much (apart from D2). I think tae has two osts and one with jin and his small acting stunt in 2016. None of the other members have any solo stuff even though jk and tae are the most popular members followed by jimin. It's the less popular members that are getting these solos because they have more confidence in their song writing to publish their work. Tae and JK have been working on their mixtapes for so long but still didn't release them because they aren't that confident in their works. I don't know if twice write any music or not but maybe the members just don't want to write a solo mixtape.
At the end of the day neither twice nor bts have any solo endorsements or brand deals yet one is called good management and the other is bad. Focusing on the group agenda has its benefits. It creates less solo stans. I mean look at bp. Sure they're the biggest gg right now but they also get the most hate within their fandoms. The girls are pitted against each other even though they are friends in rl. It also creates rifts in the fandom which can impair voting and stuff. I remember some drama with lisa solos who refused to support some voting that was ot4 since the fandom didn't help lisa with some voting. idk, it was a mess though. Both bts and bp are huge but both have very different promotional tactics that work. BTS is closer with twice and bp is more like rv. It doesn't mean bad management.
I'm sorry about the rant, I just think it's stupid for people on sns to comment on how a company promotes their group. You can disagree with things that the company does, but that doesn't mean the company is poorly managing a group.