This is more similar to iVentoy, which implements PXE boot, but is closed source. netboot.xyz, on the other hand, requires boot images to be extracted and serve

This is more similar to iVentoy, which implements PXE boot, but is closed source. netboot.xyz, on the other hand, requires boot images to be extracted and served in an iPXE-friendly format, which their CI/CD system does automatically. These solutions are good for facilitating OS installations across a fleet of computers on a network. But for system rescue purposes, Ventoy still wins. You bring your own boot images, which could be arbitrary ISOs, and don't have to worry about networking availability.