Confused on passes

Hello kakak. The destinations you mention can all be well reached from Interlaken. The entire Golden Pass between Lucerne to Montreux would be a bit too much in my opintion, but you can also travel part of this route, e.g. to Lucerne when heading for the Titlis. The best thing you can do is enter your destinations in the timetable (enter 'Kleintitlis' for the Titlis). This allows you to make an exact planning for each trip, including routes and traveling times. Enter 'Meiringen' as a via-station for the Golden Pass route between Interlaken and Lucerne. And do note that it's recommend to make seat reservations in advance.

Swiss rail passes are listed in this overview. There's a detail page on each pass, explaining exactly how to use it. These FAQ may get you started too. Since you're traveling in September, you may be eligible for the 2 for 1 offer on the 4 day Swiss Saver Pass. If you order a pass before September 5, you can profit from additional discounts. This would be the most economical option if you plan to travel with someone else.