I've used the SmartEX app with zero problems. It seems to Just Work with all regions. Pick origination and destination, then if you're within 30 days of date of travel pick your seats, then pay. That's it. My first trip to Japan was last year, I bought the first set of tickets (Tokyo - Kyoto on the Nozomi) while still at home in the US. Subsequent tickets were purchases in-country. At least on the city pairs we wanted the frequency was often enough that if the train I had picked was full there was another one a little later or earlier (but in reality it was never an issue). In-country I bought 2-3 days in advance.

Note: we did not need big bag accommodations. That might've changed the equation a bit because that space is limited. If you will have anything bigger than an airline-style carry-on you need to reserve big bag space.