I would like to ask if anyone has had luck using spacedesk wirelessly without the internet. In other words, I am looking for a method of configuring a second monitor without usb using Wifi direct or the like.
Yes, you can actually use spacedesk via a Wi-Fi direct connection.
Please refer to the Performance Tuning -> Wi-Fi Direct chapter in the user manual https://forum.spacedesk.netuser-manual/#item_510.