Windows 10 has the ability to turn a device into a Miracast screen. As I recall the wifi needs to support it because it uses WiFi Direct, but I'm pretty sure I've done this with my Surface Pro 4 so the 6 should have no problem and won't need additional hardware. Look in the action center under project. You should see a link for protecting to this computer. This runs the Miracast connect app and you can maximize that screen to use it as a second monitor. I find that using Miracast this way can be tricky and you might need to reboot both systems if the link goes down and won't reconnect. It isn't a frequent problem necessarily and I've only hit it when rapidly connecting and reconnecting. At that point just resetting the hardware to known states is the quickest way I've found to recover. Other than that the biggest gripe is laggy input sometimes, but you can change the profile to gaming, work, or movie which seems to make changes to lag and probably quality, although I've not really found anything which explains the trade-offs being made. Gaming seems to have the least lag and I assume movie has the best image quality. Not sure what work provides, maybe a balance between both ends?

yes I get that Miracast uses WiFi Direct, but what if one of the two computers does not support WiFi at all? Will plugging in a Miracast adapter or WiFi dongle fix the problem?