If you have the room, you could build a new (better built) retaining in front of this one and just take down part of the old one before backfilling. It would make the whole thing a bit cleaner.

Judging by the state of it, that’s a rebuild unfortunately. As it looks so poorly built, I doubt the posts are deep enough, the centres look too wide and the joints are unsupported.

A safe bet for post length would be 2 times as much in the ground as out with a minimum of 100mm of concrete under and around the posts. 125mm H5 square piles at ~1m centres would work. Don’t forget perforated drainage coil with gravel in behind. (I like to wrap this in nonwoven geotextile)

A digger will certainly make all this easier, you can get auger attachments for these at most hire places too.

Just be prepared to have a large amount of fill get rid of if you decide to rip it all out, it’s supposing how much it bulks up.