Hi @Matt Snead,
Thanks for your reply. I installed the Intel Graphics Command Center from the Microsoft Store and the option you mentioned in your post doesn't even show up (actually the "Power" tab under "System" is missing altogether). Another dead end...
I thought perhaps this option only appears in more recent models, but then I searched through Intel's support and it lists my iGPU (Iris Plus 640) as supposedly supporting this option. This Intel support page (https://www.intel.com/content/www/us/en/support/articles/000057194/graphics.html) has a note saying "Panel self refresh is available only for embedded DisplayPort 1.3 (eDP 1.3)." The fact is that I don't see the Panel Self Refresh option or even the Power tab. Any suggestions?