I just saw a video on the operation...and it wasn't good for the Japanese. They threw in 80% of their total forces, including all the behind the lines personnel, which meant that there was a massive vacuum that the ChiComs flooded into and set up their base of operations. Ichi-Go was a total Hail Mary operation that while it did basically sow the seeds of the collapse of the KMT also ensured the ChiComs ran the country and would have gutted Japanese logistics and ability to function if and when Mao gave the word to start and uprising.
It could be launched earlier IIRC, but would have effectively just multiplied the Japanese problems by several orders of magnitude by eliminating the main threat to the ChiComs and ceding them control over the countryside and infrastructure.