Ikuta still has her clothing line, in fact I think she launched something new just recently.
Akane is still young, 20, and she seems to be interested in photography. Not sure if she'd start university for it, but she's a clever girl, so I wouldn't be too surprised if she left for something outside of entertainment business. In fact, I wish they'd let her work more on merchandise, maybe create her own group photobook. To give her exposure and experience. Make it digital if you don't want to spend money on prints.
Yokoyan is a bit of a surprise bag. Sometimes I wonder if she'd go back to a normal life, or start as a youtuber or something. Don't get me wrong, personally I don't see Yokoyan leaving since she actually gets a bit here and there in the group. This is not like (the) Haruna(s) or Kanon who had to fight for crumbs. And even Oharu made it after MM
Miki leaving would be the most frustrating thing ever. Since 12th gen debuted she's been said to be the next-in-line singer, but now we've got Rio and Mei who are also pushing. Nobody knows how Rairi will come through, and now they're probably gonna add even more girls (and hopefully some that can sing!).
Generally I do agree with your timeline, I fear that Akane would be up there before Maria though (or rather, her being the 12th gen that'll leave before her senpai). Because she is so young and still can do something else. Girls like Mizuki, Ayumi or even Sakura don't know the business outside of H!P and probably never will get past Up Front. But I think the earlier you leave the easier it is to adapt other work policies.
Speaking of Maria, while I do think she could easily adapt outside of MM at the moment (as a gravure model, in variety, idk) I don't see her leaving. She needs a big offer or else she's going to go the Sayumi route.
Talking of skill set/entertainment footprint... what did Chii have to offer? She's a good dancer, but not on the level of professional dancing. In fact, I wonder if even Kaede can make it in dancing. Even Riho who went to the US to study dance focuses mostly on her music career with a bit of acting. And both of them are older than Riho when she left. I guess professional dancing is one of those careers that you have to do continuously, like competitive sports. Being an idol doesn't really add anything to it.