Take http://spheres5e.wikidot.com/using-spheres-of-power but make it for pf2e. Also intended to be used with Essence Casting?
Take inspiration from kineticist. Possibly make kineticist work like the rest of the system instead at some point.
Entering a circle should give you access to some broad cantrip and kinesis-like ability that gets stronger, take inspiration from that one Cantrips expanded homebrew Ilija used for 5e Dreams.
Spells will need to be categorised in new ways, especially with the loss of schools of magic. Still divide the spheres into the 4 magic traditions? Remember that Arcane is Mind/Matter, Primal is Matter/Life, Divine is Life/Spirit, and Occult is Spirit/Mind.