A very quick search for where carbon fiber steerer tubes break basically confirmed what if already guessed. The most common place is right below the stem due to over tightening and the second most common is the upper bearing where the tube exits the top of the headset.

Keep in mind that on most bikes, that spot is the same place or almost the same place, so it's not like some super detailed study.

Of note, I've broken exactly one carbon fork, way back in the early days, where the fork was epoxied to the crown. Post mortem on it sure looked like with some epoxy and a rolled pin or two, I could have put it back together stronger than before. I chose to make a lamp out of it instead.

Seems like the two best options would be 1. Get an extra long expansion plug that's long enough to span the distance from a little below the top of the headset to just below the top of the stem. OR #2, use a small expander plug deeper than the top of the headset and fill the remaining space with epoxy/jb weld with the bolt in place to allow.

Both add weight and complexity to the setup. Which method really depends on how permanent your stem height is.