Disclaimer: I am not a legal professional (in any jurisdiction), and this is not any form of an official advice.
But I've happened to work as "engineer in licensing and copy activation" for one of the major FOSS vendors, and then being a licensing compliance officer in another large-ish FOSS project; and here are my 2¢:
You are indeed in violation of GPL2 when building on top of GPL2-licensed code and giving it a non-compatible license. And PPL is explicitly non-compatible, since it restricts the "four freedoms of FOSS" for some actors (as despicable as they might be), while GPL is explicitly restricting any form of judgement when providing the four freedoms.
On a more general note — I appreciate the spirit and the intention behind your licensing choice, but unfortunately in practice this would mean severe impediment for hokusai adoption.
Like, it would be technically hard/impossible to include hokusai and derived works into any software packets distribution: they both technically and by policy are usually restricted to "clear, well-known licenses" (which typically means OSI/FSF-recognized ones), and it might not even be possible to express your licensing choice when putting the code up there.
And who would pick a GUI toolkit to build with, when the resulting code can't even be published in the package repos of their Linux distro of choice?
On a more fundamental level (and I start feeling out of my depth here a little bit) — Creative Commons licenses and their derivatives are usually not a great fit for software code. Eg right here in the comments someone already mentioned that it's not clear what constitutes "use" of your codebase — and indeed, the text of your license mostly focuses on Reproduction, Distribution and Adaptation.
If I build and html-based webapp on hokusai, run it on my own server, and you open the page — is this Distribution or Reproduction, for whom of us?
Even worse, if hokusai is being used in a startup which assigned shares (not options) to its founding engineers, but not to everyone — is it a "worker-owned business" or not?
What about an exploitive business which frames itself as a coop/NGO/whatever, but frames it actual workers as contractors (Uber-style)? Legally it might pass the benchmark outlined in PPL, but in practice it might be even more exploitative than just hiring someone at the labor market rates. Same with an org which exploits unpaid interns or volunteers...