What about that time in the 1500 when they made plans to conquer Korea and China
Both would have been essentially impossible due to numerous reasons. Korea failed because of logistics in the form of widespread localized resistance across the peninsula, which were mostly financed by the navy under Yi Sun-shin, not to mention that by the time that the Chinese intervened in late 1592 in order to aid its closest vassal, plausibility was out of the window. Even if Japan had somehow managed to pull off an impossible occupation, sporadic resistance would still continue to occur across the peninsula, tying up Japanese forces, while an incursion into China would have bankrupted the Japanese treasury, and may well have led to Japan's complete devastation by an invasion from China.
The main issue with Japanese expansion in any scenario is that although it was somewhat geographically close to Korea, China, and the Ryukyus, allowing it to establish stable trading relations for more than a millennia, it would have been virtually impossible to consistently supply a significant amount of troops invading the mainland, although this also meant that foreigners invading the archipelago wouldn't fare any better either. For similar reasons, it would be extremely unlikely for Japan to expand into and retain islands across the Pacific, as it would come into contact with European colonial powers, so it would be more profitable to trade with them indirectly through the Ryukyus, as it did IOTL.