Ah, apologies. I ought to have phrased it as '[Japan only invaded Korea] Technically once pre-1900, under Toyotomi Hideyoshi, who *among other reasons* needed an outlet to deal with the warrior class...' in the first place. Doubtless, there were other considerations, like Hideyoshi's own ambitions/megalomania, lack of legitimacy from low birth, his late lord Oda Nobunaga's dreams of further conquests, etc. but, again, those aren't factors in the 1700s and aligns with my point that Japan has only ever militarily invaded the Korean peninsula and attacked its armies for the sake of annexing it once in the last thousand years (or in all of history, possibly).Side-note: I don't personally believe the idea that Hideyoshi engineered the whole Imjin war to deplete the ranks of the samurai. The war was prosecuted with full vigor, and he had to endure one particularly humiliating incident where he came close to accidentally swearing fealty to the Chinese emperor, due to weird kabuki deception between his own diplomats and the Chinese ones. He actually ripped off the robes they had given him in front of his court and repudiated everyone involved, which I can't believe he would have submitted himself to if he was doing all this just to vent off excess manpower.
Again, it's not a usual thing for Japan to attack the mainland. Historically, they've been politically and militarily uninvolved with China and Korea for the vast majority of the last thousand years. The Imjin War and the 1800-1900s were exceptions rather than the norm.
Ah, that's more dependent on court politics. I'm not the best versed in Joseon court intrigue but King Yeongjo's reign seemed to be manageable. Then the monarchy lost all authority and the Andong Kims pretty much ran the country, with isolationism being the name of the game for the majority of the 1800s. That said, Yeongjo banned Catholicism outright, so I doubt the Joseon court would make concerted efforts to modernise, especially with foreign influences. The Joseon didn't exactly see the Japanese in the best light, after all (vastly underestimated them prior to the Imjin War and stuck by the Qing even after Japan went through the Meiji Restoration), so it's not exactly likely they'd try to follow their example without tremendous pressure to do so (like after Japan beat up the Qing and forced the Joseon out of the Qing tributary system, which led to the creation of the short lived Korean Empire. Anything short of that and enough of the Joseon court would be resistant to radical reform that nothing would get done).I would imagine them being ignorant, maybe even oblivious to the entire change, until the Japanese Attacka them.
But, I would imagine the Koreans being more aware of the situation, and maybe even they try to modernize as well