According to JR Central, the peak of crowding for this summer holiday period will be August 10 for outbound trains and August 17 for inbound ones. The following data are from this site Shinkansen Timetable’s independent survey. The seat availability changes from moment to moment, but they may be of some help to find remaining seats.
JR Central, JR West and JR Kyushu reported seat availability of Shinkansen trains during the summer holiday period from August 9 to 18.
Special Tsubame services on Kyushu Shinkansen will be available on the day of firework festivals, on August 5 for Chikugogawa Firework Festival (第360回 筑後川花火大会) and August 16 for Sendaigawa Firework festival (令和元年記念 第61回川内川花火大会).
JR Kyushu started Go! Waku Waku Trip with MICKEY Project (「Go! Waku Waku Trip with MICKEY」プロジェクト) to promote travel on Kyushu Shinkansen from this May. The centerpiece of this project is the travel of the JR Kyushu Waku Waku Trip Shinkansen (JR九州 Waku Waku Trip新幹線) which is painted lively big Mickey Mouse on its vehicle. This special painted Series 800 car set will run between Hakata – Kagoshima-Chuo as Sakura and Tsubame trains on Kyushu Shinkansen from May 17. The additional Series 800 car set named JR Kyushu Waku Waku Trip Shinkansen Mickey Mouse and Minnie Mouse Design (JR九州 Waku Waku Trip新幹線 ミッキーマウス＆ミニーマウスデザイン) painted lively big Mickey Mouse and Minnie Mouse will start to run from August 1. The existing special painted cars will travel until September 1 and the additional ones will end its travel on November 27.
In June, this site Shinkansen Timetable had been visited 26 thousand times by 17 thousand users, and the page view was 72 thousand. The 35.6% in the total access was through desktop, 59.7% through mobile, 4.7% through tablet.