How to tell my PhD supervisors I want to publish without them?
Based on your account, I am surmising that you have already completed your PhD thesis, and obtained an academic position independent of your former supervisors (well done!). That being the case, you are under no obligation (whether legal, pragmatic, or ethical) whatsoever to involve your former supervisors in publication plans for work to which they did not make an author-level contribution. If you feel that (and, now that you are a Doctor of Philosophy, you should have some confidence in your academic judgement) you will advance scholarship more effectively by publishing alone or with different collaborators, you should have no qualms about doing so.

By the way, in my humanities field (in which sole-authored publications are the norm), it is perfectly acceptable, even encouraged, to publish without your PhD supervisor while still a student. I have two excellent supervisors, but it has never occurred to any of us to pursue a joint publication. In fact, funnily enough, I have recently completed a sole-authored book-chapter for a collected volume in which my principal supervisor just happens to also be writing his own sole-authored book-chapter (on a different topic, although relevant enough that I actually cite a few ideas from his chapter in mine).