I know Loreto Normanhurst actively engage past parents with a decent base/volume of people they are in contact with. You could speak to Liane (let me know if you need an introduction) about how this has historically been managed and the formal structures around it. They rebranded their community of Alumnae and Past Parents to the Normo Network when they launched their online community. Most schools I speak to very happy to include past parents with Alumni especially if they are keen on mentoring programs as it adds depth of experience / talent in the pool of mentors. Separately, I know The Kings School are wrestling with this like you based on soon to be ‘past parents’ wanting to remain connected after their (youngest) son completes their studies.
All the best working out the right solve for PLC Sydney