My speculation for reasons Nina is graduating:
1. Bad/unprofessional management and general toxic work environment at Niji EN, I think we can expect significantly more of them to not renew as they get closer to the 2 year mark
2. Money dispute, probably thinks she can make more money as indie/holo/vshojo/etc
3. I think Niji EN is way more cutthroat than anybody thinks, she may have been pressured to leave for not meeting performance targets
4. Her channel just isn't growing, too many dead subs, low CCV on a lot of streams, at this point it's better for the algo for her to reboot
5. Based on recent collabs it seems to me she gets along way better with people outside the company, particularly Kiara, may be something to her getting in Holo gen 3, as far as I know Holo EN doesn't have a 30 something woman, just a bunch of mid to late 20s, could be a whole new demo for them