Honestly, compared to Steve, Reimu honestly feels like the kind of character that Sakurai would take just as much, if not more, time to develop, but he would still have better confidence in how to properly pull her mechanics off. I mean, the only reason he got his first job at HAL Laboratories and started his game development career was because of Gall Force, a SHMUP game made by that same company. Compared to a sandbox survival game like Minecraft, Sakurai seems much more familiar with the SHMUP genre overall. Now the fact that Sakurai will surpass Steve in terms of complexity with Reimu is untrue until proven otherwise, but if he wants to aim that high, he would have better confidence with, say, a constantly-active Zamasu/Magneto-style flight mechanic on all the stages, as opposed to Steve's Mine/Craft/Create Block Special Move, because the SHMUP genre is more closely tied to his career.
As for the negotiations themselves, absolutely, the amount of time it would take to negotiate Touhou in Smash would be about equivalent to snapping your fingers. Touhou, at this rate, is such a close equivalent to being public domain it's not even funny. The longer part would be development, as 7NATOR said. Whether it be Reimu's gimmicks, how to balance her moveset, her appearance, what stage would be used, and how abundant the music would be, that should be the most time-consuming, some much more than others. I'm probably gonna look like a parrot for bringing up Challenger Pack 4 again, but Sakurai probably took a long time with that pack, having to decide which songs to source from, which songs to remix, how many stage cameos there are, how the stage mechanic works, implementing the charge input Terry's Up-B uses, transplanting Ryu and Ken's FGC inputs and expanding upon them, and which Spirits to include in his board. You can say what you will about Terry's DLC Pack, but all that had to have taken longer than usual, only being beaten by Steve at most.