Britain had beaten China twice in the opium war, if they can kick China's ass, they can kick Japan's way more times.
And Britain's conquest of it was way more than 4 years before then. Also, if China had just finished fighting basically a civil war (the tai ping rebellion) it's very weak and vulnerable. Japan that had been industrializing for like a decade before this could pull it off.
wym by garrison in this context?
Emperor Tewodros II attempted to modernize Ethiopia by centralizing power, reforming the military, and introducing Western technology. Witnessing the colonization of neighboring African territories by European powers, particularly the French conquest of Algeria and British expansion in East Africa, likely instilled a sense of urgency in Tewodros II to modernize Ethiopia's military and administration. He recognized the need for a stronger state to resist potential European encroachment, without any other country knocking on his doors. What was happening to the Sikh empire could have done the same to Japan.
ok, maybe we can make it the 2nd opium war or something. Also the dutch or the jesuit missionaries could have spread word around in Japan
1. And the 2nd time happened after Perry's Black Ships threatened to burn its way to Edo. Also, it's not like it was the first few times China had been beaten and forced to give concessions to a foreign power (the Liao and Jin with the Song, the Later Jin with the Ming). The Chinese also didn't consider the 1st or 2nd Opium Wars to be the start of a Century of Humiliation. The extent of the exploitation and other disasters couldn't have been predicted in 1830 and barely could be foreseen in the 1850s, which is when Japan finally opened up under foreign pressure.
2. Japan took 26 years from the Meiji Restoration (1868) to the 1st Sino-Japanese War (1894). And that's after another 16 years between Perry (1853) and Meiji. They didn't spend those 42 years idly. They were investing in infrastructure, military reform, and dealing with internal dissent stemming from balancing concessions to foreign power with national pride. A mere decade would've been too short to make all the preparations and the British and French were still mucking around in China. Japan couldn't defeat the Russians, who had a much weaker navy, after 50 years of modernization, so they'd be limited or entirely rebuffed by the Anglo-French forces if they seem to threaten Anglo-French interests.
3. Conquered lands need to be policed to watch out for internal threats and garrisoned with armed forced to protect against external threats (and also more intense internal threats like rebellions). Japan's subjugation of Korea was rooted in the Japanese government's perceived need to protect Japan against Russia, and so they sent troops into Korea. Those troops take money and manpower to maintain and the more land taken, the more expensive such garrisons become. All the resources dedicated to garrisoning are resources not spent on improving the metropole, which was part of why Japan didn't impose on the mainland for 42 years after Perry's expedition.
4. Seeing neighboring nations getting conquered is not the same as maybe hearing about some unknown nation 5000 km away (Nagasaki to Lahore) getting conquered by another nation you don't know much about. One is approaching the level of a personal threat, the other is news that might be hearsay, for all they knew. For another comparison, the US didn't declare war on the Japanese Empire, despite the latter's expansionism in US stomping grounds in the 1930s, until Japan attacked US territory directly. And the US had way more investment in China than Japan had in India at that point (or any point, really).
As for your example in Ethiopia, Tewodros II hearing about British incursions into East Africa instilling urgency in him makes sense because his nation was in East Africa and he knew about the other East African nations and at least a bit of their capabilities. But he, nor any other Ethiopian leader, wouldn't have cared about the Europeans conquering the Americas and India despite how those showcased European potential and ambitions for conquering overseas lands and the technology that they used for those purposes. It'd be the same for Japan, and it is was in OTL. There was anxiety about the 1st Opium War and those did contribute to a push to modernize artillery and stop firing on foreign ships (to prevent another Arrow Incident), but there wasn't enough to force the rapid modernization Perry elicited.
5. But why would the Japanese care about some Northeastern Indian empire? They didn't have direct trade with them (the Sikh Empire didn't even have a coastline) or direct communication or diplomacy. It's like hearing news about a 4rd cousin twice removed. You might recognize the name, but chances are there's not going to be much of an emotional response. Having Korea get beaten into submission and/or China getting Opium War'd earlier along with more forceful European presence in Japan would be more productive for your purposes. By that argument, the Chinese should've started wising up after the British conquest of India (which is much closer to them than it is to Japan). It took another 20-30 years, 2 Opium Wars, and the Taiping Rebellion + other rebellions for an actual concerted effort by the Qing government to reform the army and industralize to match the Europeans though.