About items such as the Märklin 7391 or 7389 buffer stops that clip on the rails, but that are not tracks per se, it would still be nice if there was a way to have then snap on to the track (any position for a buffer) and stay connected when we move the track.

Just groupping the elements is not really a good solution, since there are transformations (movements) that change the relative position of the grouped elements, and since that also prevents some operations on the track (since the group is identified as an heterogeneous group).

This could apply to some other elements such as the signals, and why not, perhaps the buildings too.