This is the most plausible explanation. I've seen idiot takes like Aqua refusing to be sacrificed to the chinese market, etc.
Most likely it's just a difference of priority. It's undeniable that there's a lot of work the talents put behind the scenes unrelated to streaming, and Aqua's probably exhausted from all those work. She just wanna stream and play games, but Cover's rapid expansion doesn't allow for that. She took a hiatus last year, and probably feels bad if she took it again.
Judging by Peko's comments, she's been planning to leave for months, and that other members tried to convince her to stay. There's also been negotiations on both parts, so we can pretty much rule out nefarious theories. Going by Occam's Razor, most likely she simply wants to focus on streaming & playing games.