The short answer is that it's impossible to really disentangle Japan's engagement with China from its war in the Pacific. Without the war in China, Japan would have been unlikely to have launched its campaigns against the Western powers at all, since the need to bring the war in China to a conclusion favorable to Japan was a key aspect of why Japan went south in the first place.

With that being said, it is a quit difficult question to answer. By the time the wider Pacific War broke out in 1941, Nationalist Chinese had indeed suffered severe losses, both in terms of troops and territory. The pre-war core of German trained troops had been all but wiped out in the fighting for Shanghai, while Japanese forced had seized the majority of China's infrastructure (both railways and ports) and its most well developed economic areas. However, despite all these losses, China remained in the war, and by 1940, the Japanese had largely failed to make further headway into the remaining Nationalist held areas. This would remain the case until the Ichi-Go offensive of 1944. The sheer scale of the conflict in China means that I find it very difficult to describe the war as "inconsequential". It certainly was not inconsequential for the millions of Chinese who were suffering or dead due to direct or indirect impacts of the war.

The War in China undoubtedly tied up a considerable amount of the Imperial Japanese Army for the entirety of the war. In December 1941, the Japanese Army had 51 divisions available. The grand Japanese offensive that overran the European and American colonial holdings in Southeast Asia used 10 IJA divisions. Another 13 divisions were in Manchuria, guarding against the Soviet Union, or preparing to take advantage of a total Soviet collapse, while 27 divisions were engaged in China. Had those divisions been available for redeployment out into the Pacific, it would undoubtedly have played a factor in the war, even if just by increasing the number of Japanese troops that could have been deployed in island garrisons or in key battles. The ongoing conflict in China also meant that Japanese efforts to exploit the resources and industry of China were continually hampered, with the entire war being a net drain on Japanese resources. Even prior to the outbreak of the wider Pacific War, the drain on resources from the war in China had forced the Japanese to ration rice, fuel, and other commodities within Japan itself, along with starving the civilian economy of resources to support the continued war effort and build up for future conflicts. On a smaller scale, the Imperial Japanese Navy's long range bombers had provided the main strength for Japanese strategic bombing of cities in the Chinese interior, and the need for those forces in the Pacific basically brought an end to long range bombing in China, as the Army didn't have aircraft to replace them.

It's difficult to come to any counterfactual conclusions on what might have happened had Japan either won the war in China or never started it, and counterfactual history is not something we do on r/AskHistorians. However, I hope this has helped to illustrate the scale of the Second Sino-Japanese War, in terms of the resources it consumed from Japan. Ultimately, the Japanese war effort against the United States was always a desperate gamble, and it was not the presence or lack thereof of Army divisions that tilted the tables so firmly against Japan. However, that is not to say that the war in China was "inconsequential" in any respect. The world we live in today is still directly impacted by the decisions of Japanese leadership in the 1930s and 40s.

Please feel free to ask any follow ups.