It's just a string winder with a pivot.  How can it be so complicated?  Something has to support the parts & be manufacturable.  Facing the rapidly escalating complexity of this, model airplane rubber bands, clutches, confuser fans, & animal leashes got appealing.  Computer fans have the hall effect sensors & brushless motor control in a convenient package, but if they stall they shut down.  They also don't make as much torque as hoped.  A model airplane rubber band would have to stick out the side of the spool & be very bulky.

After a few prints trying to dial in the tolerances, along came the smallest animal leash made, the Flexi.  It actually compresses 10ft of string in less space than the motorized retractor.

Pictured with robot for scale, it's smaller than a motorized retractor can hope to be.

It's not serviceable without destruction.

The special sauce is a self contained spring + spool.  There's nothing special going on to keep the string untangled besides constant tension.  The spring was permanently enclosed by a press fitted blue thing.

The end of the spring slips down a channel in the shaft.

The spool rotates entirely around plastic bearings.  There's a ratchet mechanism which acts as a break.  Ditching the ratchet could further reduce the size.  An axial spring keeps it from rattling.  It's liberally bathed in grease to dampen it.  

The great task is creating a new enclosure for it, attaching magnets to it for an encoder, detecting sideways movement.

Note how sections of plastic were manually ground away to seemingly smooth out rough parts of the rotation.