score:6
There will be essentially no impact. The coronation will take place on the grounds of the Imperial Palace, very much off limits to the public, and subsequent celebrations consist of a banquet limited to a few hundred invited guests only.
In general, the Japanese Imperial family both holds itself at arm's length from and is kept at a distance by the general public, and there's no tradition of royal pomp and public hysteria to the extent you'd see in (say) the UK or Thailand.
More reading: https://www.nippon.com/en/japan-topics/c06107/japan-to-spend-record-%C2%A516-6-billion-on-coronation-of-new-emperor.html