Note that you do not have to worry about there not being “room” on the train. It is possible that more people board a train then there are seats, but that a train has to leave passengers behind is so rare that it makes the national news here when it does.
For Interlaken - Strasbourg you do not need any reservations. Just hop on the train in Interlaken at a time convenient to you, and travel, all the way to Strasbourg, changing trains in Basel.
For Garmish - Interlaken you do not need to reserve seats, but you should for at least the Munich - Zurich train, and that you can do on the Bahn.de site.