Retaining wall repair and conversion to tiers
Not sure that the cause here is "no backfill" as it looks full in back of the wall. Also, although it is difficult to tell for sure from pictures, the wood does not look rotten.

It seems as though you have 2 problems:


the wall timbers were not adequately tied together vertically and are now moving horizontally independently.
the wall was not adequately secured with some type of 'tie-back" scheme.


Both issues could be resolved, but whether it is a "huge job" depends on perspective. A professional contractor could likely force the timbers back in place (using mechanical or hydraulic jacks or other means) and auger through them from the top down, hammering in rebar to tie them together.

Also, one could trench up-slope and install tie-backs or use one of the several available "retro-fit" tie back gadgets.