This setup allows you to start a single Sway remote desktop session on a virtual display directly from Moonlight by sending the Wake PC request, waiting a few seconds and connecting to a fresh new desktop. Or you can just have a desktop running constantly in the background. This requires no dummy plugs or input devices attached to the host, but will also allow you to sometimes log in locally and and have that session available on sunshine too. Some of this stuff probably also works with other wlroots-based compositors with small changes.
I'm assuming you've first read and memorized the Sunshine documentation, it probably includes some very important stuff I didn't put here. The stuff about udev rules is probably important, cap_sys_admin not so much.
This is not supposed to be a complete guide to set up headless sunshine streaming and I assume you already know how to