@Gauvain Ratchet crossbows can't be used with one hand. They were too long and heavy for that. There was a light crossbow for use by riders, but these didn't have anything like the power of the ratchet crossbows because they were drawn using the power of the leg and just one arm. The analog of the one-handed cavalry crossbow was actually the pistol, which started life as a cavalry weapon and had significantly more stopping power than the light crossbow to start with.
Also, Japanese cavalrymen didn't need reins to control their horses. They were after all fighting with two-handed polearms and longbows to begin with. They used their legs to apply control just like every other East Asian horseman in the period.
There is absolutely no advantage in using ratchet crossbow in the Sengoku era. To begin with, Japan didn't even have the right type of spring steel available domestically until the 19th century. They would have had to make high-draw crossbows using composites just like the Chinese and other mainlanders, and it took between 4 months to half a year just to cure the glue to bond the disparate materials together. Production would have been a total nightmare compared to the mass-produced Tanegashima matchlocks. The author just needs a way for Shizuko to contribute technology to sengoku-era warfare but chose the wrong item to do it with.
You know what Shizuko could have contributed? Basic medical knowledge. 90% of battle casualties die AFTER the battle. Now, in the case of the Japanese headhunting had a lot to do with it, but providing basic medical services for battle survivors would save many lives on the winning side.
Or just paper armor. The Qing Dynasty in China switched into using paper armor in the mid-17th century due to encounters with firearms-wielding "barbarians". They discovered that multiple layers of soft materials worked better against arrows and bullets than steel did. This is of course how we modern people armor ourselves against modern firearms today.