I’ve tried several free apps to do head/face tracking. Opentrack is used along with AI Track or Smoothtrack (inexpensive) or some other head tracking software. These free or low cost apps may do what you want, especially if you have a compatible web cam already.
I have had to use Droidcam, which allows a phone to be used as a webcam, because I don’t have a web cam. I think the combination of three apps may slow things down. While the Droidcam/AI Track/Open Track combination works, it results in slow response on my system. You likely have a faster system than mine.
I now have three monitors, which provide a wide view, so head tracking is no longer the advantage it was with only one monitor.
I’d recommend trying some of this software before you invest in Tobii just to see how it would work on your system. Youtube has quite a few reviews of these apps and how to configure them for MSFS. Opentrack and AI Track are free downloads on Github. There may be free “eye track” software, too.
Prior to Tobii Eye Tracker, I believe Tobii technology tracked a small reflecting material located at the side of the head. Face/head trackers do not require wearing such a device. People argue the advantages and disadvantages of each. Some have made a DIY reflector and claim it works as well as the same tech Tobii. I’m not familiar with eye versus face/head tracking advantages and disadvantages.
Maybe you’ve seen this thread, but if not: Head/Face Tracking Alternatives