As much as I enjoy shitting on Niji and doom'n'glooming them, NijiJP is still seen positively in the JP market, so continuing that relation is not toxic by any means for PhaseJP. I am aware of recent events like a well known NijiJP talent graduating, immediately reincarnating, and then getting tweeted by her colleagues. However you have to understand how resilient the JP audience is to change. Any of these revelations about the contract means nothing to JP fans. Continuing to collab with Sayu is fine but if bringing her in as an Invader shuts off PhaseJP from any future collabs with NijiJP, it honestly wouldn't be worth it.
Phase is doing just fine and growing in the EN market and it doesn't need Sayu to join, meanwhile Sayu is doing just fine as an indie. The new PhaseJP gen has managed to break ground but they still have a ways to go to grow in the JP market, jeopardizing for no reason would be silly.