Alright, restarted a Hosakawa, the largest daimyo. I have all of japan except 3-4 provinces, and even though I have the "unite japan" decision, whenever I take it those provinces dont defect to me and I am unable to sign peace agreements to get them. So I'll have to take them while I'm a daimyo and then unite.
Its going to take me 10-20 years though. BB limit is a terrible thing. I tried going over it and just blitzing... I could do it, but I really do not want to be dealing with stacks of 10 rebels every week and my reputation + legitimacy plummeting.
Interesting. I united when two Daimyo were not under my control, and I was capable of just using Reconquest five years later on.
Also, remember that you automatically inherit unions and vassals when forming Japan.
I united as Shimazu before Nationalism actually was available for the Daimyos, it took me quite a bit of time because of infamy, but not cascading alliances (much, those still were a pain).
Iirc the trick was building a powerful fleet, then getting the islands in the south protected by the narrow straits, and day-one declaring on the OPM daimyo right next to you. Also remember to colonize Taiwan instantly and Hokkaido later on.
There is a bit of luck involved in getting a good alliance partner or two to distract people a bit (I had Uesugi), but once I had control of the starting island, I was capable of grabbing the gold province and was pretty good to go. Although, the wars after that still were pretty evil.