I had the same lategame issues with taking the tech lead from the US and big European civs, and I think there just needs to be some big war for them to get bogged down in in the late Industrial or early Global eras to break some of those really high-value trade routes and divert hammers into units. I'm still fairly new to the mod as well and I enjoy playing as China and Japan so I'm hoping to take another few stabs at this. All of my attempts on Monarch/Normal have struggled with UHV2 or UHV3, and I'm considering trying Epic speed next.
By the way, what does your opening look like for Japan? I'm a big fan of swapping into Republic/Citizenship/Manorialism/Merchant Trade, pumping workers from Kyoto at size 2, and founding 2 more cities in a tight triangle 2E of Kyoto and NE-NE of Kyoto on one of the Silks. The starting civics feel pretty lackluster given the available food (crucially, Rice paddy fields are not farms and thus do not get -1 food from Republic) and land, I find it helpful to finish UHV1 early with a Great Artist, and I don't mind the tight pattern because it's all core pop in anticipation of UHV2. Perhaps a 2-city approach is more efficient but I feel so bad with an unused starting Settler

My first tech target is Feudalism for Himeji with Deification, which with a Theatre and Walls/Castle starts making steady progress towards UHV1. It's really a shame that it's nigh impossible to justify a 4th city to get a Buddhist cathedral for the additional +50% culture multiplier. One really important thing that's easy to overlook with Himeji is that you actually need to take the promotions before the game updates a unit's level. After finishing the wonder I swap to Monasticism/Conquest and start stuffing Himeji with cheap level 3s and then start teching toward Caravels to meet the world and enable the Modernization tech discounts.
EDIT: I was tinkering a bit with the opening on Monarch-Normal and I found time for 3 Spies, which were really impressively useful. Since they can move on roads and ignore open borders requirements, I was able to get one all the way to Portugal, which turned my beeline this game to Cartography into Education, Gunpowder, and Doctrine. It also allowed me to get the circumnavigation bonus with my first Caravel in 1535, which will be very useful further into the game.
Also, despite not generating any Great Artists at odds ranging from 20-33% out of 7 Great People so far (4 Scientists, 2 Engineers, and a Statesman), I'm on pace to finish UHV1 even if my next two Great People also aren't Artists at 42 and 49%. This is made possible by my early Himeji, which is adding 80 base culture per turn from units, mostly cheap level 3 Skirmishers from Barracks+Conquest. I also swapped back to Vassalage after finishing the discounted Citizenship buildings, so I'm paying very low unit expenses, and even with the penalty from Republic + Vassalage I have plenty of net stability from the other civics (Republic/Vassalage/Manorialism/Regulated Trade/Monasticism/Conquest). I tried first mining over the Tea hill and Silk hill at Nagano, and it worked nicely in that there's enough time and worker turns to turn them back into plantations before the happiness is needed, and an extra 1/3/1 tile is excellent when building all of the discounted Redistribution/Citizenship infrastructure.
I have two Settlers lined up for after UHV1, and I'm nearly finished with the Medieval techs and thinking of how to get into the world's good graces for Modernization. Fortunately there's a Buddhist bloc to trade with, but I was thinking of taking a page from the China UHV3 strategy book and settling Manila for an Islam spread, then converting to that and looking to foster trade with the Ottomans, Moors, and Abbasids.
EDIT: I was going to stop playing after completing UHV1, but it turns out I did get the Great Artist, and also triggered the Aztecs conquerors, which is certainly new for me, considering I haven't played as any of the European civs yet

. It's kind of silly that I actually need to delay capturing any Aztec cities so that they don't screw up UHV1.