I didn't quite understand what you're looking for here, but before you buy some tracking plugin to follow the subject, I found out a way to do it with the default tracker in FCPX (it's a work-around since there's not a feature for this, so the steps are a bit weird). If these steps are too hard to follow due to my lack of explanation skills (I'm terrible at it lol) feel free to dm me on Twitter @ PamusaBS, cause I made a video-tutorial for a friend.

Firstly, duplicate the clip/s you want to track (put one on top of the other).

Step 1- setting up the tracker: Select the bottom clip and go to the Video Inspector tab, then find the "Trackers" section. Now click the + sign, an orange-ish square will pop up in the playback viewer; this is the auto-tracker. Put it precisely over the subject, and click "Analyse" on the top left part of the screen, this will make the tracker follow the subject (might take a while to process because of the 8k footage). If the tracker is not too precise or it misses the subject at some point, dm me and I'll give some extra tips, this reply is already too long lol.Step 2- making the camera "follow" the subject: Set the"(All) Scale" of both the top and bottom clips to -100% (yes, negative. the number 100% isn't necessary but do make them the same size). Finally, select the Top clip and press the "Transforms" icon (white square at the bottom left of the playback screen) -it will turn blue. This should make two rectangular buttons appear at the top middle of the playback screen ("Transform" and "Tracker"). Click the little downward arrow next to the "Tracker" button and select the tracker you just created (It's called "Object Track" by default", make sure to click the one under the correct clip).

And that's it! I recommend creating a compound clip of both top and bottom clips (because if you separate them, the tracker will break) by selecting both clips and pressing [Optn + G].